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Convert content from the ext4 wiki to Documentation rst files so it is

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more likely to be updated as we add new features to ext4.  Add 64-bit
 timestamp support to ext4's superblock fields.  And the usual bug
 fixes and cleanups, including a Spectre gadget fixup and some
 hardening against maliciously corrupted file systems.
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Merge tag 'ext4_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4

Pull ext4 updates from Ted Ts'o:

 - Convert content from the ext4 wiki to Documentation rst files so it
   is more likely to be updated as we add new features to ext4.

 - Add 64-bit timestamp support to ext4's superblock fields.

 - ... and the usual bug fixes and cleanups, including a Spectre gadget
   fixup and some hardening against maliciously corrupted file systems.

* tag 'ext4_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4: (34 commits)
  ext4: remove unneeded variable "err" in ext4_mb_release_inode_pa()
  ext4: improve code readability in ext4_iget()
  ext4: fix spectre gadget in ext4_mb_regular_allocator()
  ext4: check for NUL characters in extended attribute's name
  ext4: use ext4_warning() for sb_getblk failure
  ext4: fix race when setting the bitmap corrupted flag
  ext4: reset error code in ext4_find_entry in fallback
  ext4: handle layout changes to pinned DAX mappings
  dax: dax_layout_busy_page() warn on !exceptional
  docs: fix up the obviously obsolete bits in the new ext4 documentation
  docs: add new ext4 superblock time extension fields
  docs: create filesystem internal section
  ext4: use swap macro in mext_page_double_lock
  ext4: check allocation failure when duplicating "data" in ext4_remount()
  ext4: fix warning message in ext4_enable_quotas()
  ext4: super: extend timestamps to 40 bits
  jbd2: replace current_kernel_time64 with ktime equivalent
  ext4: use timespec64 for all inode times
  ext4: use ktime_get_real_seconds for i_dtime
  ext4: use 64-bit timestamps for mmp_time
  ...
This commit is contained in:
Linus Torvalds 2018-08-13 22:34:47 -07:00
commit 10f3e23f07
42 changed files with 4038 additions and 152 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
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@ -34,7 +34,7 @@ needs_sphinx = '1.3'
# Add any Sphinx extension module names here, as strings. They can be
# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
# ones.
extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include', 'cdomain', 'kfigure']
extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include', 'cdomain', 'kfigure', 'sphinx.ext.ifconfig']
# The name of the math extension changed on Sphinx 1.4
if major == 1 and minor > 3:

144
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@ -1,6 +1,8 @@
.. SPDX-License-Identifier: GPL-2.0
Ext4 Filesystem
===============
========================
General Information
========================
Ext4 is an advanced level of the ext3 filesystem which incorporates
scalability and reliability enhancements for supporting large filesystems
@ -11,35 +13,28 @@ Mailing list: linux-ext4@vger.kernel.org
Web site: http://ext4.wiki.kernel.org
1. Quick usage instructions:
===========================
Quick usage instructions
========================
Note: More extensive information for getting started with ext4 can be
found at the ext4 wiki site at the URL:
http://ext4.wiki.kernel.org/index.php/Ext4_Howto
found at the ext4 wiki site at the URL:
http://ext4.wiki.kernel.org/index.php/Ext4_Howto
- Compile and install the latest version of e2fsprogs (as of this
writing version 1.41.3) from:
http://sourceforge.net/project/showfiles.php?group_id=2406
or
- The latest version of e2fsprogs can be found at:
https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
or
http://sourceforge.net/project/showfiles.php?group_id=2406
or grab the latest git repository from:
git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
- Note that it is highly important to install the mke2fs.conf file
that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
you have edited the /etc/mke2fs.conf file installed on your system,
you will need to merge your changes with the version from e2fsprogs
1.41.x.
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
- Create a new filesystem using the ext4 filesystem type:
# mke2fs -t ext4 /dev/hda1
# mke2fs -t ext4 /dev/hda1
Or to configure an existing ext3 filesystem to support extents:
@ -50,10 +45,6 @@ Note: More extensive information for getting started with ext4 can be
# tune2fs -I 256 /dev/hda1
(Note: we currently do not have tools to convert an ext4
filesystem back to ext3; so please do not do try this on production
filesystems.)
- Mounting:
# mount -t ext4 /dev/hda1 /wherever
@ -75,10 +66,11 @@ Note: More extensive information for getting started with ext4 can be
the filesystem with a large journal can also be helpful for
metadata-intensive workloads.
2. Features
===========
Features
========
2.1 Currently available
Currently Available
-------------------
* ability to use filesystems > 16TB (e2fsprogs support not available yet)
* extent format reduces metadata overhead (RAM, IO for access, transactions)
@ -103,31 +95,15 @@ Note: More extensive information for getting started with ext4 can be
[1] Filesystems with a block size of 1k may see a limit imposed by the
directory hash tree having a maximum depth of two.
2.2 Candidate features for future inclusion
* online defrag (patches available but not well tested)
* reduced mke2fs time via lazy itable initialization in conjunction with
the uninit_bg feature (capability to do this is available in e2fsprogs
but a kernel thread to do lazy zeroing of unused inode table blocks
after filesystem is first mounted is required for safety)
There are several others under discussion, whether they all make it in is
partly a function of how much time everyone has to work on them. Features like
metadata checksumming have been discussed and planned for a bit but no patches
exist yet so I'm not sure they're in the near-term roadmap.
The big performance win will come with mballoc, delalloc and flex_bg
grouping of bitmaps and inode tables. Some test results available here:
- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
3. Options
==========
Options
=======
When mounting an ext4 filesystem, the following option are accepted:
(*) == default
======================= =======================================================
Mount Option Description
======================= =======================================================
ro Mount filesystem read only. Note that ext4 will
replay the journal (and thus write to the
partition) even when mounted "read only". The
@ -387,33 +363,38 @@ i_version Enable 64-bit inode version support. This option is
dax Use direct access (no page cache). See
Documentation/filesystems/dax.txt. Note that
this option is incompatible with data=journal.
======================= =======================================================
Data Mode
=========
There are 3 different data modes:
* writeback mode
In data=writeback mode, ext4 does not journal data at all. This mode provides
a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
mode - metadata journaling. A crash+recovery can cause incorrect data to
appear in files which were written shortly before the crash. This mode will
typically provide the best ext4 performance.
In data=writeback mode, ext4 does not journal data at all. This mode provides
a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
mode - metadata journaling. A crash+recovery can cause incorrect data to
appear in files which were written shortly before the crash. This mode will
typically provide the best ext4 performance.
* ordered mode
In data=ordered mode, ext4 only officially journals metadata, but it logically
groups metadata information related to data changes with the data blocks into a
single unit called a transaction. When it's time to write the new metadata
out to disk, the associated data blocks are written first. In general,
this mode performs slightly slower than writeback but significantly faster than journal mode.
In data=ordered mode, ext4 only officially journals metadata, but it logically
groups metadata information related to data changes with the data blocks into
a single unit called a transaction. When it's time to write the new metadata
out to disk, the associated data blocks are written first. In general, this
mode performs slightly slower than writeback but significantly faster than
journal mode.
* journal mode
data=journal mode provides full data and metadata journaling. All new data is
written to the journal first, and then to its final location.
In the event of a crash, the journal can be replayed, bringing both data and
metadata into a consistent state. This mode is the slowest except when data
needs to be read from and written to disk at the same time where it
outperforms all others modes. Enabling this mode will disable delayed
allocation and O_DIRECT support.
data=journal mode provides full data and metadata journaling. All new data is
written to the journal first, and then to its final location. In the event of
a crash, the journal can be replayed, bringing both data and metadata into a
consistent state. This mode is the slowest except when data needs to be read
from and written to disk at the same time where it outperforms all others
modes. Enabling this mode will disable delayed allocation and O_DIRECT
support.
/proc entries
=============
@ -425,10 +406,12 @@ Information about mounted ext4 file systems can be found in
in table below.
Files in /proc/fs/ext4/<devname>
..............................................................................
================ =======
File Content
================ =======
mb_groups details of multiblock allocator buddy cache of free blocks
..............................................................................
================ =======
/sys entries
============
@ -439,28 +422,30 @@ Information about mounted ext4 file systems can be found in
/sys/fs/ext4/dm-0). The files in each per-device directory are shown
in table below.
Files in /sys/fs/ext4/<devname>
(see also Documentation/ABI/testing/sysfs-fs-ext4)
..............................................................................
File Content
Files in /sys/fs/ext4/<devname>:
(see also Documentation/ABI/testing/sysfs-fs-ext4)
============================= =================================================
File Content
============================= =================================================
delayed_allocation_blocks This file is read-only and shows the number of
blocks that are dirty in the page cache, but
which do not have their location in the
filesystem allocated yet.
inode_goal Tuning parameter which (if non-zero) controls
inode_goal Tuning parameter which (if non-zero) controls
the goal inode used by the inode allocator in
preference to all other allocation heuristics.
This is intended for debugging use only, and
should be 0 on production systems.
inode_readahead_blks Tuning parameter which controls the maximum
inode_readahead_blks Tuning parameter which controls the maximum
number of inode table blocks that ext4's inode
table readahead algorithm will pre-read into
the buffer cache
lifetime_write_kbytes This file is read-only and shows the number of
lifetime_write_kbytes This file is read-only and shows the number of
kilobytes of data that have been written to this
filesystem since it was created.
@ -508,7 +493,7 @@ Files in /sys/fs/ext4/<devname>
in the file system. If there is not enough space
for the reserved space when mounting the file
mount will _not_ fail.
..............................................................................
============================= =================================================
Ioctls
======
@ -518,8 +503,10 @@ through the system call interfaces. The list of all Ext4 specific ioctls are
shown in the table below.
Table of Ext4 specific ioctls
..............................................................................
Ioctl Description
============================= =================================================
Ioctl Description
============================= =================================================
EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
The ioctl argument is an integer bitfield, with
bit values described in ext4.h. This ioctl is an
@ -610,8 +597,7 @@ Table of Ext4 specific ioctls
normal user by accident.
The data blocks of the previous boot loader
will be associated with the given inode.
..............................................................................
============================= =================================================
References
==========

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@ -0,0 +1,17 @@
.. SPDX-License-Identifier: GPL-2.0
===============
ext4 Filesystem
===============
General usage and on-disk artifacts writen by ext4. More documentation may
be ported from the wiki as time permits. This should be considered the
canonical source of information as the details here have been reviewed by
the ext4 community.
.. toctree::
:maxdepth: 5
:numbered:
ext4
ondisk/index

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@ -0,0 +1,44 @@
.. SPDX-License-Identifier: GPL-2.0
About this Book
===============
This document attempts to describe the on-disk format for ext4
filesystems. The same general ideas should apply to ext2/3 filesystems
as well, though they do not support all the features that ext4 supports,
and the fields will be shorter.
**NOTE**: This is a work in progress, based on notes that the author
(djwong) made while picking apart a filesystem by hand. The data
structure definitions should be current as of Linux 4.18 and
e2fsprogs-1.44. All comments and corrections are welcome, since there is
undoubtedly plenty of lore that might not be reflected in freshly
created demonstration filesystems.
License
-------
This book is licensed under the terms of the GNU Public License, v2.
Terminology
-----------
ext4 divides a storage device into an array of logical blocks both to
reduce bookkeeping overhead and to increase throughput by forcing larger
transfer sizes. Generally, the block size will be 4KiB (the same size as
pages on x86 and the block layer's default block size), though the
actual size is calculated as 2 ^ (10 + ``sb.s_log_block_size``) bytes.
Throughout this document, disk locations are given in terms of these
logical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of
convenience, the logical block size will be referred to as
``$block_size`` throughout the rest of the document.
When referenced in ``preformatted text`` blocks, ``sb`` refers to fields
in the super block, and ``inode`` refers to fields in an inode table
entry.
Other References
----------------
Also see http://www.nongnu.org/ext2-doc/ for quite a collection of
information about ext2/3. Here's another old reference:
http://wiki.osdev.org/Ext2

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@ -0,0 +1,56 @@
.. SPDX-License-Identifier: GPL-2.0
Block and Inode Allocation Policy
---------------------------------
ext4 recognizes (better than ext3, anyway) that data locality is
generally a desirably quality of a filesystem. On a spinning disk,
keeping related blocks near each other reduces the amount of movement
that the head actuator and disk must perform to access a data block,
thus speeding up disk IO. On an SSD there of course are no moving parts,
but locality can increase the size of each transfer request while
reducing the total number of requests. This locality may also have the
effect of concentrating writes on a single erase block, which can speed
up file rewrites significantly. Therefore, it is useful to reduce
fragmentation whenever possible.
The first tool that ext4 uses to combat fragmentation is the multi-block
allocator. When a file is first created, the block allocator
speculatively allocates 8KiB of disk space to the file on the assumption
that the space will get written soon. When the file is closed, the
unused speculative allocations are of course freed, but if the
speculation is correct (typically the case for full writes of small
files) then the file data gets written out in a single multi-block
extent. A second related trick that ext4 uses is delayed allocation.
Under this scheme, when a file needs more blocks to absorb file writes,
the filesystem defers deciding the exact placement on the disk until all
the dirty buffers are being written out to disk. By not committing to a
particular placement until it's absolutely necessary (the commit timeout
is hit, or sync() is called, or the kernel runs out of memory), the hope
is that the filesystem can make better location decisions.
The third trick that ext4 (and ext3) uses is that it tries to keep a
file's data blocks in the same block group as its inode. This cuts down
on the seek penalty when the filesystem first has to read a file's inode
to learn where the file's data blocks live and then seek over to the
file's data blocks to begin I/O operations.
The fourth trick is that all the inodes in a directory are placed in the
same block group as the directory, when feasible. The working assumption
here is that all the files in a directory might be related, therefore it
is useful to try to keep them all together.
The fifth trick is that the disk volume is cut up into 128MB block
groups; these mini-containers are used as outlined above to try to
maintain data locality. However, there is a deliberate quirk -- when a
directory is created in the root directory, the inode allocator scans
the block groups and puts that directory into the least heavily loaded
block group that it can find. This encourages directories to spread out
over a disk; as the top-level directory/file blobs fill up one block
group, the allocators simply move on to the next block group. Allegedly
this scheme evens out the loading on the block groups, though the author
suspects that the directories which are so unlucky as to land towards
the end of a spinning drive get a raw deal performance-wise.
Of course if all of these mechanisms fail, one can always use e4defrag
to defragment files.

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@ -0,0 +1,191 @@
.. SPDX-License-Identifier: GPL-2.0
Extended Attributes
-------------------
Extended attributes (xattrs) are typically stored in a separate data
block on the disk and referenced from inodes via ``inode.i_file_acl*``.
The first use of extended attributes seems to have been for storing file
ACLs and other security data (selinux). With the ``user_xattr`` mount
option it is possible for users to store extended attributes so long as
all attribute names begin with “user”; this restriction seems to have
disappeared as of Linux 3.0.
There are two places where extended attributes can be found. The first
place is between the end of each inode entry and the beginning of the
next inode entry. For example, if inode.i\_extra\_isize = 28 and
sb.inode\_size = 256, then there are 256 - (128 + 28) = 100 bytes
available for in-inode extended attribute storage. The second place
where extended attributes can be found is in the block pointed to by
``inode.i_file_acl``. As of Linux 3.11, it is not possible for this
block to contain a pointer to a second extended attribute block (or even
the remaining blocks of a cluster). In theory it is possible for each
attribute's value to be stored in a separate data block, though as of
Linux 3.11 the code does not permit this.
Keys are generally assumed to be ASCIIZ strings, whereas values can be
strings or binary data.
Extended attributes, when stored after the inode, have a header
``ext4_xattr_ibody_header`` that is 4 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- h\_magic
- Magic number for identification, 0xEA020000. This value is set by the
Linux driver, though e2fsprogs doesn't seem to check it(?)
The beginning of an extended attribute block is in
``struct ext4_xattr_header``, which is 32 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- h\_magic
- Magic number for identification, 0xEA020000.
* - 0x4
- \_\_le32
- h\_refcount
- Reference count.
* - 0x8
- \_\_le32
- h\_blocks
- Number of disk blocks used.
* - 0xC
- \_\_le32
- h\_hash
- Hash value of all attributes.
* - 0x10
- \_\_le32
- h\_checksum
- Checksum of the extended attribute block.
* - 0x14
- \_\_u32
- h\_reserved[2]
- Zero.
The checksum is calculated against the FS UUID, the 64-bit block number
of the extended attribute block, and the entire block (header +
entries).
Following the ``struct ext4_xattr_header`` or
``struct ext4_xattr_ibody_header`` is an array of
``struct ext4_xattr_entry``; each of these entries is at least 16 bytes
long. When stored in an external block, the ``struct ext4_xattr_entry``
entries must be stored in sorted order. The sort order is
``e_name_index``, then ``e_name_len``, and finally ``e_name``.
Attributes stored inside an inode do not need be stored in sorted order.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_u8
- e\_name\_len
- Length of name.
* - 0x1
- \_\_u8
- e\_name\_index
- Attribute name index. There is a discussion of this below.
* - 0x2
- \_\_le16
- e\_value\_offs
- Location of this attribute's value on the disk block where it is stored.
Multiple attributes can share the same value. For an inode attribute
this value is relative to the start of the first entry; for a block this
value is relative to the start of the block (i.e. the header).
* - 0x4
- \_\_le32
- e\_value\_inum
- The inode where the value is stored. Zero indicates the value is in the
same block as this entry. This field is only used if the
INCOMPAT\_EA\_INODE feature is enabled.
* - 0x8
- \_\_le32
- e\_value\_size
- Length of attribute value.
* - 0xC
- \_\_le32
- e\_hash
- Hash value of attribute name and attribute value. The kernel doesn't
update the hash for in-inode attributes, so for that case this value
must be zero, because e2fsck validates any non-zero hash regardless of
where the xattr lives.
* - 0x10
- char
- e\_name[e\_name\_len]
- Attribute name. Does not include trailing NULL.
Attribute values can follow the end of the entry table. There appears to
be a requirement that they be aligned to 4-byte boundaries. The values
are stored starting at the end of the block and grow towards the
xattr\_header/xattr\_entry table. When the two collide, the overflow is
put into a separate disk block. If the disk block fills up, the
filesystem returns -ENOSPC.
The first four fields of the ``ext4_xattr_entry`` are set to zero to
mark the end of the key list.
Attribute Name Indices
~~~~~~~~~~~~~~~~~~~~~~
Logically speaking, extended attributes are a series of key=value pairs.
The keys are assumed to be NULL-terminated strings. To reduce the amount
of on-disk space that the keys consume, the beginning of the key string
is matched against the attribute name index. If a match is found, the
attribute name index field is set, and matching string is removed from
the key name. Here is a map of name index values to key prefixes:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Name Index
- Key Prefix
* - 0
- (no prefix)
* - 1
- “user.”
* - 2
- “system.posix\_acl\_access”
* - 3
- “system.posix\_acl\_default”
* - 4
- “trusted.”
* - 6
- “security.”
* - 7
- “system.” (inline\_data only?)
* - 8
- “system.richacl” (SuSE kernels only?)
For example, if the attribute key is “user.fubar”, the attribute name
index is set to 1 and the “fubar” name is recorded on disk.
POSIX ACLs
~~~~~~~~~~
POSIX ACLs are stored in a reduced version of the Linux kernel (and
libacl's) internal ACL format. The key difference is that the version
number is different (1) and the ``e_id`` field is only stored for named
user and group ACLs.

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.. SPDX-License-Identifier: GPL-2.0
Bigalloc
--------
At the moment, the default size of a block is 4KiB, which is a commonly
supported page size on most MMU-capable hardware. This is fortunate, as
ext4 code is not prepared to handle the case where the block size
exceeds the page size. However, for a filesystem of mostly huge files,
it is desirable to be able to allocate disk blocks in units of multiple
blocks to reduce both fragmentation and metadata overhead. The
`bigalloc <Bigalloc>`__ feature provides exactly this ability. The
administrator can set a block cluster size at mkfs time (which is stored
in the s\_log\_cluster\_size field in the superblock); from then on, the
block bitmaps track clusters, not individual blocks. This means that
block groups can be several gigabytes in size (instead of just 128MiB);
however, the minimum allocation unit becomes a cluster, not a block,
even for directories. TaoBao had a patchset to extend the “use units of
clusters instead of blocks” to the extent tree, though it is not clear
where those patches went-- they eventually morphed into “extent tree v2”
but that code has not landed as of May 2015.

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@ -0,0 +1,28 @@
.. SPDX-License-Identifier: GPL-2.0
Block and inode Bitmaps
-----------------------
The data block bitmap tracks the usage of data blocks within the block
group.
The inode bitmap records which entries in the inode table are in use.
As with most bitmaps, one bit represents the usage status of one data
block or inode table entry. This implies a block group size of 8 \*
number\_of\_bytes\_in\_a\_logical\_block.
NOTE: If ``BLOCK_UNINIT`` is set for a given block group, various parts
of the kernel and e2fsprogs code pretends that the block bitmap contains
zeros (i.e. all blocks in the group are free). However, it is not
necessarily the case that no blocks are in use -- if ``meta_bg`` is set,
the bitmaps and group descriptor live inside the group. Unfortunately,
ext2fs\_test\_block\_bitmap2() will return '0' for those locations,
which produces confusing debugfs output.
Inode Table
-----------
Inode tables are statically allocated at mkfs time. Each block group
descriptor points to the start of the table, and the superblock records
the number of inodes per group. See the section on inodes for more
information.

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.. SPDX-License-Identifier: GPL-2.0
Layout
------
The layout of a standard block group is approximately as follows (each
of these fields is discussed in a separate section below):
.. list-table::
:widths: 1 1 1 1 1 1 1 1
:header-rows: 1
* - Group 0 Padding
- ext4 Super Block
- Group Descriptors
- Reserved GDT Blocks
- Data Block Bitmap
- inode Bitmap
- inode Table
- Data Blocks
* - 1024 bytes
- 1 block
- many blocks
- many blocks
- 1 block
- 1 block
- many blocks
- many more blocks
For the special case of block group 0, the first 1024 bytes are unused,
to allow for the installation of x86 boot sectors and other oddities.
The superblock will start at offset 1024 bytes, whichever block that
happens to be (usually 0). However, if for some reason the block size =
1024, then block 0 is marked in use and the superblock goes in block 1.
For all other block groups, there is no padding.
The ext4 driver primarily works with the superblock and the group
descriptors that are found in block group 0. Redundant copies of the
superblock and group descriptors are written to some of the block groups
across the disk in case the beginning of the disk gets trashed, though
not all block groups necessarily host a redundant copy (see following
paragraph for more details). If the group does not have a redundant
copy, the block group begins with the data block bitmap. Note also that
when the filesystem is freshly formatted, mkfs will allocate “reserve
GDT block” space after the block group descriptors and before the start
of the block bitmaps to allow for future expansion of the filesystem. By
default, a filesystem is allowed to increase in size by a factor of
1024x over the original filesystem size.
The location of the inode table is given by ``grp.bg_inode_table_*``. It
is continuous range of blocks large enough to contain
``sb.s_inodes_per_group * sb.s_inode_size`` bytes.
As for the ordering of items in a block group, it is generally
established that the super block and the group descriptor table, if
present, will be at the beginning of the block group. The bitmaps and
the inode table can be anywhere, and it is quite possible for the
bitmaps to come after the inode table, or for both to be in different
groups (flex\_bg). Leftover space is used for file data blocks, indirect
block maps, extent tree blocks, and extended attributes.
Flexible Block Groups
---------------------
Starting in ext4, there is a new feature called flexible block groups
(flex\_bg). In a flex\_bg, several block groups are tied together as one
logical block group; the bitmap spaces and the inode table space in the
first block group of the flex\_bg are expanded to include the bitmaps
and inode tables of all other block groups in the flex\_bg. For example,
if the flex\_bg size is 4, then group 0 will contain (in order) the
superblock, group descriptors, data block bitmaps for groups 0-3, inode
bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining
space in group 0 is for file data. The effect of this is to group the
block metadata close together for faster loading, and to enable large
files to be continuous on disk. Backup copies of the superblock and
group descriptors are always at the beginning of block groups, even if
flex\_bg is enabled. The number of block groups that make up a flex\_bg
is given by 2 ^ ``sb.s_log_groups_per_flex``.
Meta Block Groups
-----------------
Without the option META\_BG, for safety concerns, all block group
descriptors copies are kept in the first block group. Given the default
128MiB(2^27 bytes) block group size and 64-byte group descriptors, ext4
can have at most 2^27/64 = 2^21 block groups. This limits the entire
filesystem size to 2^21 ∗ 2^27 = 2^48bytes or 256TiB.
The solution to this problem is to use the metablock group feature
(META\_BG), which is already in ext3 for all 2.6 releases. With the
META\_BG feature, ext4 filesystems are partitioned into many metablock
groups. Each metablock group is a cluster of block groups whose group
descriptor structures can be stored in a single disk block. For ext4
filesystems with 4 KB block size, a single metablock group partition
includes 64 block groups, or 8 GiB of disk space. The metablock group
feature moves the location of the group descriptors from the congested
first block group of the whole filesystem into the first group of each
metablock group itself. The backups are in the second and last group of
each metablock group. This increases the 2^21 maximum block groups limit
to the hard limit 2^32, allowing support for a 512PiB filesystem.
The change in the filesystem format replaces the current scheme where
the superblock is followed by a variable-length set of block group
descriptors. Instead, the superblock and a single block group descriptor
block is placed at the beginning of the first, second, and last block
groups in a meta-block group. A meta-block group is a collection of
block groups which can be described by a single block group descriptor
block. Since the size of the block group descriptor structure is 32
bytes, a meta-block group contains 32 block groups for filesystems with
a 1KB block size, and 128 block groups for filesystems with a 4KB
blocksize. Filesystems can either be created using this new block group
descriptor layout, or existing filesystems can be resized on-line, and
the field s\_first\_meta\_bg in the superblock will indicate the first
block group using this new layout.
Please see an important note about ``BLOCK_UNINIT`` in the section about
block and inode bitmaps.
Lazy Block Group Initialization
-------------------------------
A new feature for ext4 are three block group descriptor flags that
enable mkfs to skip initializing other parts of the block group
metadata. Specifically, the INODE\_UNINIT and BLOCK\_UNINIT flags mean
that the inode and block bitmaps for that group can be calculated and
therefore the on-disk bitmap blocks are not initialized. This is
generally the case for an empty block group or a block group containing
only fixed-location block group metadata. The INODE\_ZEROED flag means
that the inode table has been initialized; mkfs will unset this flag and
rely on the kernel to initialize the inode tables in the background.
By not writing zeroes to the bitmaps and inode table, mkfs time is
reduced considerably. Note the feature flag is RO\_COMPAT\_GDT\_CSUM,
but the dumpe2fs output prints this as “uninit\_bg”. They are the same
thing.

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.. SPDX-License-Identifier: GPL-2.0
+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| i.i\_block Offset | Where It Points |
+=====================+==============================================================================================================================================================================================================================+
| 0 to 11 | Direct map to file blocks 0 to 11. |
+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 12 | Indirect block: (file blocks 12 to (``$block_size`` / 4) + 11, or 12 to 1035 if 4KiB blocks) |
| | |
| | +------------------------------+--------------------------------------------------------------------+ |
| | | Indirect Block Offset | Where It Points | |
| | +==============================+====================================================================+ |
| | | 0 to (``$block_size`` / 4) | Direct map to (``$block_size`` / 4) blocks (1024 if 4KiB blocks) | |
| | +------------------------------+--------------------------------------------------------------------+ |
+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 13 | Double-indirect block: (file blocks ``$block_size``/4 + 12 to (``$block_size`` / 4) ^ 2 + (``$block_size`` / 4) + 11, or 1036 to 1049611 if 4KiB blocks) |
| | |
| | +--------------------------------+---------------------------------------------------------------------------------------------------------+ |
| | | Double Indirect Block Offset | Where It Points | |
| | +================================+=========================================================================================================+ |
| | | 0 to (``$block_size`` / 4) | Map to (``$block_size`` / 4) indirect blocks (1024 if 4KiB blocks) | |
| | | | | |
| | | | +------------------------------+--------------------------------------------------------------------+ | |
| | | | | Indirect Block Offset | Where It Points | | |
| | | | +==============================+====================================================================+ | |
| | | | | 0 to (``$block_size`` / 4) | Direct map to (``$block_size`` / 4) blocks (1024 if 4KiB blocks) | | |
| | | | +------------------------------+--------------------------------------------------------------------+ | |
| | +--------------------------------+---------------------------------------------------------------------------------------------------------+ |
+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 14 | Triple-indirect block: (file blocks (``$block_size`` / 4) ^ 2 + (``$block_size`` / 4) + 12 to (``$block_size`` / 4) ^ 3 + (``$block_size`` / 4) ^ 2 + (``$block_size`` / 4) + 12, or 1049612 to 1074791436 if 4KiB blocks) |
| | |
| | +--------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+ |
| | | Triple Indirect Block Offset | Where It Points | |
| | +================================+================================================================================================================================================+ |
| | | 0 to (``$block_size`` / 4) | Map to (``$block_size`` / 4) double indirect blocks (1024 if 4KiB blocks) | |
| | | | | |
| | | | +--------------------------------+---------------------------------------------------------------------------------------------------------+ | |
| | | | | Double Indirect Block Offset | Where It Points | | |
| | | | +================================+=========================================================================================================+ | |
| | | | | 0 to (``$block_size`` / 4) | Map to (``$block_size`` / 4) indirect blocks (1024 if 4KiB blocks) | | |
| | | | | | | | |
| | | | | | +------------------------------+--------------------------------------------------------------------+ | | |
| | | | | | | Indirect Block Offset | Where It Points | | | |
| | | | | | +==============================+====================================================================+ | | |
| | | | | | | 0 to (``$block_size`` / 4) | Direct map to (``$block_size`` / 4) blocks (1024 if 4KiB blocks) | | | |
| | | | | | +------------------------------+--------------------------------------------------------------------+ | | |
| | | | +--------------------------------+---------------------------------------------------------------------------------------------------------+ | |
| | +--------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+ |
+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

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.. SPDX-License-Identifier: GPL-2.0
Blocks
------
ext4 allocates storage space in units of “blocks”. A block is a group of
sectors between 1KiB and 64KiB, and the number of sectors must be an
integral power of 2. Blocks are in turn grouped into larger units called
block groups. Block size is specified at mkfs time and typically is
4KiB. You may experience mounting problems if block size is greater than
page size (i.e. 64KiB blocks on a i386 which only has 4KiB memory
pages). By default a filesystem can contain 2^32 blocks; if the '64bit'
feature is enabled, then a filesystem can have 2^64 blocks.
For 32-bit filesystems, limits are as follows:
.. list-table::
:widths: 1 1 1 1 1
:header-rows: 1
* - Item
- 1KiB
- 2KiB
- 4KiB
- 64KiB
* - Blocks
- 2^32
- 2^32
- 2^32
- 2^32
* - Inodes
- 2^32
- 2^32
- 2^32
- 2^32
* - File System Size
- 4TiB
- 8TiB
- 16TiB
- 256PiB
* - Blocks Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Inodes Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Block Group Size
- 8MiB
- 32MiB
- 128MiB
- 32GiB
* - Blocks Per File, Extents
- 2^32
- 2^32
- 2^32
- 2^32
* - Blocks Per File, Block Maps
- 16,843,020
- 134,480,396
- 1,074,791,436
- 4,398,314,962,956 (really 2^32 due to field size limitations)
* - File Size, Extents
- 4TiB
- 8TiB
- 16TiB
- 256TiB
* - File Size, Block Maps
- 16GiB
- 256GiB
- 4TiB
- 256TiB
For 64-bit filesystems, limits are as follows:
.. list-table::
:widths: 1 1 1 1 1
:header-rows: 1
* - Item
- 1KiB
- 2KiB
- 4KiB
- 64KiB
* - Blocks
- 2^64
- 2^64
- 2^64
- 2^64
* - Inodes
- 2^32
- 2^32
- 2^32
- 2^32
* - File System Size
- 16ZiB
- 32ZiB
- 64ZiB
- 1YiB
* - Blocks Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Inodes Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Block Group Size
- 8MiB
- 32MiB
- 128MiB
- 32GiB
* - Blocks Per File, Extents
- 2^32
- 2^32
- 2^32
- 2^32
* - Blocks Per File, Block Maps
- 16,843,020
- 134,480,396
- 1,074,791,436
- 4,398,314,962,956 (really 2^32 due to field size limitations)
* - File Size, Extents
- 4TiB
- 8TiB
- 16TiB
- 256TiB
* - File Size, Block Maps
- 16GiB
- 256GiB
- 4TiB
- 256TiB
Note: Files not using extents (i.e. files using block maps) must be
placed within the first 2^32 blocks of a filesystem. Files with extents
must be placed within the first 2^48 blocks of a filesystem. It's not
clear what happens with larger filesystems.

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.. SPDX-License-Identifier: GPL-2.0
Checksums
---------
Starting in early 2012, metadata checksums were added to all major ext4
and jbd2 data structures. The associated feature flag is metadata\_csum.
The desired checksum algorithm is indicated in the superblock, though as
of October 2012 the only supported algorithm is crc32c. Some data
structures did not have space to fit a full 32-bit checksum, so only the
lower 16 bits are stored. Enabling the 64bit feature increases the data
structure size so that full 32-bit checksums can be stored for many data
structures. However, existing 32-bit filesystems cannot be extended to
enable 64bit mode, at least not without the experimental resize2fs
patches to do so.
Existing filesystems can have checksumming added by running
``tune2fs -O metadata_csum`` against the underlying device. If tune2fs
encounters directory blocks that lack sufficient empty space to add a
checksum, it will request that you run ``e2fsck -D`` to have the
directories rebuilt with checksums. This has the added benefit of
removing slack space from the directory files and rebalancing the htree
indexes. If you \_ignore\_ this step, your directories will not be
protected by a checksum!
The following table describes the data elements that go into each type
of checksum. The checksum function is whatever the superblock describes
(crc32c as of October 2013) unless noted otherwise.
.. list-table::
:widths: 1 1 4
:header-rows: 1
* - Metadata
- Length
- Ingredients
* - Superblock
- \_\_le32
- The entire superblock up to the checksum field. The UUID lives inside
the superblock.
* - MMP
- \_\_le32
- UUID + the entire MMP block up to the checksum field.
* - Extended Attributes
- \_\_le32
- UUID + the entire extended attribute block. The checksum field is set to
zero.
* - Directory Entries
- \_\_le32
- UUID + inode number + inode generation + the directory block up to the
fake entry enclosing the checksum field.
* - HTREE Nodes
- \_\_le32
- UUID + inode number + inode generation + all valid extents + HTREE tail.
The checksum field is set to zero.
* - Extents
- \_\_le32
- UUID + inode number + inode generation + the entire extent block up to
the checksum field.
* - Bitmaps
- \_\_le32 or \_\_le16
- UUID + the entire bitmap. Checksums are stored in the group descriptor,
and truncated if the group descriptor size is 32 bytes (i.e. ^64bit)
* - Inodes
- \_\_le32
- UUID + inode number + inode generation + the entire inode. The checksum
field is set to zero. Each inode has its own checksum.
* - Group Descriptors
- \_\_le16
- If metadata\_csum, then UUID + group number + the entire descriptor;
else if gdt\_csum, then crc16(UUID + group number + the entire
descriptor). In all cases, only the lower 16 bits are stored.

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.. SPDX-License-Identifier: GPL-2.0
Directory Entries
-----------------
In an ext4 filesystem, a directory is more or less a flat file that maps
an arbitrary byte string (usually ASCII) to an inode number on the
filesystem. There can be many directory entries across the filesystem
that reference the same inode number--these are known as hard links, and
that is why hard links cannot reference files on other filesystems. As
such, directory entries are found by reading the data block(s)
associated with a directory file for the particular directory entry that
is desired.
Linear (Classic) Directories
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By default, each directory lists its entries in an “almost-linear”
array. I write “almost” because it's not a linear array in the memory
sense because directory entries are not split across filesystem blocks.
Therefore, it is more accurate to say that a directory is a series of
data blocks and that each block contains a linear array of directory
entries. The end of each per-block array is signified by reaching the
end of the block; the last entry in the block has a record length that
takes it all the way to the end of the block. The end of the entire
directory is of course signified by reaching the end of the file. Unused
directory entries are signified by inode = 0. By default the filesystem
uses ``struct ext4_dir_entry_2`` for directory entries unless the
“filetype” feature flag is not set, in which case it uses
``struct ext4_dir_entry``.
The original directory entry format is ``struct ext4_dir_entry``, which
is at most 263 bytes long, though on disk you'll need to reference
``dirent.rec_len`` to know for sure.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- inode
- Number of the inode that this directory entry points to.
* - 0x4
- \_\_le16
- rec\_len
- Length of this directory entry. Must be a multiple of 4.
* - 0x6
- \_\_le16
- name\_len
- Length of the file name.
* - 0x8
- char
- name[EXT4\_NAME\_LEN]
- File name.
Since file names cannot be longer than 255 bytes, the new directory
entry format shortens the rec\_len field and uses the space for a file
type flag, probably to avoid having to load every inode during directory
tree traversal. This format is ``ext4_dir_entry_2``, which is at most
263 bytes long, though on disk you'll need to reference
``dirent.rec_len`` to know for sure.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- inode
- Number of the inode that this directory entry points to.
* - 0x4
- \_\_le16
- rec\_len
- Length of this directory entry.
* - 0x6
- \_\_u8
- name\_len
- Length of the file name.
* - 0x7
- \_\_u8
- file\_type
- File type code, see ftype_ table below.
* - 0x8
- char
- name[EXT4\_NAME\_LEN]
- File name.
.. _ftype:
The directory file type is one of the following values:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0
- Unknown.
* - 0x1
- Regular file.
* - 0x2
- Directory.
* - 0x3
- Character device file.
* - 0x4
- Block device file.
* - 0x5
- FIFO.
* - 0x6
- Socket.
* - 0x7
- Symbolic link.
In order to add checksums to these classic directory blocks, a phony
``struct ext4_dir_entry`` is placed at the end of each leaf block to
hold the checksum. The directory entry is 12 bytes long. The inode
number and name\_len fields are set to zero to fool old software into
ignoring an apparently empty directory entry, and the checksum is stored
in the place where the name normally goes. The structure is
``struct ext4_dir_entry_tail``:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- det\_reserved\_zero1
- Inode number, which must be zero.
* - 0x4
- \_\_le16
- det\_rec\_len
- Length of this directory entry, which must be 12.
* - 0x6
- \_\_u8
- det\_reserved\_zero2
- Length of the file name, which must be zero.
* - 0x7
- \_\_u8
- det\_reserved\_ft
- File type, which must be 0xDE.
* - 0x8
- \_\_le32
- det\_checksum
- Directory leaf block checksum.
The leaf directory block checksum is calculated against the FS UUID, the
directory's inode number, the directory's inode generation number, and
the entire directory entry block up to (but not including) the fake
directory entry.
Hash Tree Directories
~~~~~~~~~~~~~~~~~~~~~
A linear array of directory entries isn't great for performance, so a
new feature was added to ext3 to provide a faster (but peculiar)
balanced tree keyed off a hash of the directory entry name. If the
EXT4\_INDEX\_FL (0x1000) flag is set in the inode, this directory uses a
hashed btree (htree) to organize and find directory entries. For
backwards read-only compatibility with ext2, this tree is actually
hidden inside the directory file, masquerading as “empty” directory data
blocks! It was stated previously that the end of the linear directory
entry table was signified with an entry pointing to inode 0; this is
(ab)used to fool the old linear-scan algorithm into thinking that the
rest of the directory block is empty so that it moves on.
The root of the tree always lives in the first data block of the
directory. By ext2 custom, the '.' and '..' entries must appear at the
beginning of this first block, so they are put here as two
``struct ext4_dir_entry_2``\ s and not stored in the tree. The rest of
the root node contains metadata about the tree and finally a hash->block
map to find nodes that are lower in the htree. If
``dx_root.info.indirect_levels`` is non-zero then the htree has two
levels; the data block pointed to by the root node's map is an interior
node, which is indexed by a minor hash. Interior nodes in this tree
contains a zeroed out ``struct ext4_dir_entry_2`` followed by a
minor\_hash->block map to find leafe nodes. Leaf nodes contain a linear
array of all ``struct ext4_dir_entry_2``; all of these entries
(presumably) hash to the same value. If there is an overflow, the
entries simply overflow into the next leaf node, and the
least-significant bit of the hash (in the interior node map) that gets
us to this next leaf node is set.
To traverse the directory as a htree, the code calculates the hash of
the desired file name and uses it to find the corresponding block
number. If the tree is flat, the block is a linear array of directory
entries that can be searched; otherwise, the minor hash of the file name
is computed and used against this second block to find the corresponding
third block number. That third block number will be a linear array of
directory entries.
To traverse the directory as a linear array (such as the old code does),
the code simply reads every data block in the directory. The blocks used
for the htree will appear to have no entries (aside from '.' and '..')
and so only the leaf nodes will appear to have any interesting content.
The root of the htree is in ``struct dx_root``, which is the full length
of a data block:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- dot.inode
- inode number of this directory.
* - 0x4
- \_\_le16
- dot.rec\_len
- Length of this record, 12.
* - 0x6
- u8
- dot.name\_len
- Length of the name, 1.
* - 0x7
- u8
- dot.file\_type
- File type of this entry, 0x2 (directory) (if the feature flag is set).
* - 0x8
- char
- dot.name[4]
- “.\\0\\0\\0”
* - 0xC
- \_\_le32
- dotdot.inode
- inode number of parent directory.
* - 0x10
- \_\_le16
- dotdot.rec\_len
- block\_size - 12. The record length is long enough to cover all htree
data.
* - 0x12
- u8
- dotdot.name\_len
- Length of the name, 2.
* - 0x13
- u8
- dotdot.file\_type
- File type of this entry, 0x2 (directory) (if the feature flag is set).
* - 0x14
- char
- dotdot\_name[4]
- “..\\0\\0”
* - 0x18
- \_\_le32
- struct dx\_root\_info.reserved\_zero
- Zero.
* - 0x1C
- u8
- struct dx\_root\_info.hash\_version
- Hash type, see dirhash_ table below.
* - 0x1D
- u8
- struct dx\_root\_info.info\_length
- Length of the tree information, 0x8.
* - 0x1E
- u8
- struct dx\_root\_info.indirect\_levels
- Depth of the htree. Cannot be larger than 3 if the INCOMPAT\_LARGEDIR
feature is set; cannot be larger than 2 otherwise.
* - 0x1F
- u8
- struct dx\_root\_info.unused\_flags
-
* - 0x20
- \_\_le16
- limit
- Maximum number of dx\_entries that can follow this header, plus 1 for
the header itself.
* - 0x22
- \_\_le16
- count
- Actual number of dx\_entries that follow this header, plus 1 for the
header itself.
* - 0x24
- \_\_le32
- block
- The block number (within the directory file) that goes with hash=0.
* - 0x28
- struct dx\_entry
- entries[0]
- As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
.. _dirhash:
The directory hash is one of the following values:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0
- Legacy.
* - 0x1
- Half MD4.
* - 0x2
- Tea.
* - 0x3
- Legacy, unsigned.
* - 0x4
- Half MD4, unsigned.
* - 0x5
- Tea, unsigned.
Interior nodes of an htree are recorded as ``struct dx_node``, which is
also the full length of a data block:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- fake.inode
- Zero, to make it look like this entry is not in use.
* - 0x4
- \_\_le16
- fake.rec\_len
- The size of the block, in order to hide all of the dx\_node data.
* - 0x6
- u8
- name\_len
- Zero. There is no name for this “unused” directory entry.
* - 0x7
- u8
- file\_type
- Zero. There is no file type for this “unused” directory entry.
* - 0x8
- \_\_le16
- limit
- Maximum number of dx\_entries that can follow this header, plus 1 for
the header itself.
* - 0xA
- \_\_le16
- count
- Actual number of dx\_entries that follow this header, plus 1 for the
header itself.
* - 0xE
- \_\_le32
- block
- The block number (within the directory file) that goes with the lowest
hash value of this block. This value is stored in the parent block.
* - 0x12
- struct dx\_entry
- entries[0]
- As many 8-byte ``struct dx_entry`` as fits in the rest of the data block.
The hash maps that exist in both ``struct dx_root`` and
``struct dx_node`` are recorded as ``struct dx_entry``, which is 8 bytes
long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- hash
- Hash code.
* - 0x4
- \_\_le32
- block
- Block number (within the directory file, not filesystem blocks) of the
next node in the htree.
(If you think this is all quite clever and peculiar, so does the
author.)
If metadata checksums are enabled, the last 8 bytes of the directory
block (precisely the length of one dx\_entry) are used to store a
``struct dx_tail``, which contains the checksum. The ``limit`` and
``count`` entries in the dx\_root/dx\_node structures are adjusted as
necessary to fit the dx\_tail into the block. If there is no space for
the dx\_tail, the user is notified to run e2fsck -D to rebuild the
directory index (which will ensure that there's space for the checksum.
The dx\_tail structure is 8 bytes long and looks like this:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- u32
- dt\_reserved
- Zero.
* - 0x4
- \_\_le32
- dt\_checksum
- Checksum of the htree directory block.
The checksum is calculated against the FS UUID, the htree index header
(dx\_root or dx\_node), all of the htree indices (dx\_entry) that are in
use, and the tail block (dx\_tail).

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.. SPDX-License-Identifier: GPL-2.0
Dynamic Structures
==================
Dynamic metadata are created on the fly when files and blocks are
allocated to files.
.. include:: inodes.rst
.. include:: ifork.rst
.. include:: directory.rst
.. include:: attributes.rst

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.. SPDX-License-Identifier: GPL-2.0
Large Extended Attribute Values
-------------------------------
To enable ext4 to store extended attribute values that do not fit in the
inode or in the single extended attribute block attached to an inode,
the EA\_INODE feature allows us to store the value in the data blocks of
a regular file inode. This “EA inode” is linked only from the extended
attribute name index and must not appear in a directory entry. The
inode's i\_atime field is used to store a checksum of the xattr value;
and i\_ctime/i\_version store a 64-bit reference count, which enables
sharing of large xattr values between multiple owning inodes. For
backward compatibility with older versions of this feature, the
i\_mtime/i\_generation *may* store a back-reference to the inode number
and i\_generation of the **one** owning inode (in cases where the EA
inode is not referenced by multiple inodes) to verify that the EA inode
is the correct one being accessed.

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.. SPDX-License-Identifier: GPL-2.0
Global Structures
=================
The filesystem is sharded into a number of block groups, each of which
have static metadata at fixed locations.
.. include:: super.rst
.. include:: group_descr.rst
.. include:: bitmaps.rst
.. include:: mmp.rst
.. include:: journal.rst

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.. SPDX-License-Identifier: GPL-2.0
Block Group Descriptors
-----------------------
Each block group on the filesystem has one of these descriptors
associated with it. As noted in the Layout section above, the group
descriptors (if present) are the second item in the block group. The
standard configuration is for each block group to contain a full copy of
the block group descriptor table unless the sparse\_super feature flag
is set.
Notice how the group descriptor records the location of both bitmaps and
the inode table (i.e. they can float). This means that within a block
group, the only data structures with fixed locations are the superblock
and the group descriptor table. The flex\_bg mechanism uses this
property to group several block groups into a flex group and lay out all
of the groups' bitmaps and inode tables into one long run in the first
group of the flex group.
If the meta\_bg feature flag is set, then several block groups are
grouped together into a meta group. Note that in the meta\_bg case,
however, the first and last two block groups within the larger meta
group contain only group descriptors for the groups inside the meta
group.
flex\_bg and meta\_bg do not appear to be mutually exclusive features.
In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the
block group descriptor was only 32 bytes long and therefore ends at
bg\_checksum. On an ext4 filesystem with the 64bit feature enabled, the
block group descriptor expands to at least the 64 bytes described below;
the size is stored in the superblock.
If gdt\_csum is set and metadata\_csum is not set, the block group
checksum is the crc16 of the FS UUID, the group number, and the group
descriptor structure. If metadata\_csum is set, then the block group
checksum is the lower 16 bits of the checksum of the FS UUID, the group
number, and the group descriptor structure. Both block and inode bitmap
checksums are calculated against the FS UUID, the group number, and the
entire bitmap.
The block group descriptor is laid out in ``struct ext4_group_desc``.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- bg\_block\_bitmap\_lo
- Lower 32-bits of location of block bitmap.
* - 0x4
- \_\_le32
- bg\_inode\_bitmap\_lo
- Lower 32-bits of location of inode bitmap.
* - 0x8
- \_\_le32
- bg\_inode\_table\_lo
- Lower 32-bits of location of inode table.
* - 0xC
- \_\_le16
- bg\_free\_blocks\_count\_lo
- Lower 16-bits of free block count.
* - 0xE
- \_\_le16
- bg\_free\_inodes\_count\_lo
- Lower 16-bits of free inode count.
* - 0x10
- \_\_le16
- bg\_used\_dirs\_count\_lo
- Lower 16-bits of directory count.
* - 0x12
- \_\_le16
- bg\_flags
- Block group flags. See the bgflags_ table below.
* - 0x14
- \_\_le32
- bg\_exclude\_bitmap\_lo
- Lower 32-bits of location of snapshot exclusion bitmap.
* - 0x18
- \_\_le16
- bg\_block\_bitmap\_csum\_lo
- Lower 16-bits of the block bitmap checksum.
* - 0x1A
- \_\_le16
- bg\_inode\_bitmap\_csum\_lo
- Lower 16-bits of the inode bitmap checksum.
* - 0x1C
- \_\_le16
- bg\_itable\_unused\_lo
- Lower 16-bits of unused inode count. If set, we needn't scan past the
``(sb.s_inodes_per_group - gdt.bg_itable_unused)``\ th entry in the
inode table for this group.
* - 0x1E
- \_\_le16
- bg\_checksum
- Group descriptor checksum; crc16(sb\_uuid+group+desc) if the
RO\_COMPAT\_GDT\_CSUM feature is set, or crc32c(sb\_uuid+group\_desc) &
0xFFFF if the RO\_COMPAT\_METADATA\_CSUM feature is set.
* -
-
-
- These fields only exist if the 64bit feature is enabled and s_desc_size
> 32.
* - 0x20
- \_\_le32
- bg\_block\_bitmap\_hi
- Upper 32-bits of location of block bitmap.
* - 0x24
- \_\_le32
- bg\_inode\_bitmap\_hi
- Upper 32-bits of location of inodes bitmap.
* - 0x28
- \_\_le32
- bg\_inode\_table\_hi
- Upper 32-bits of location of inodes table.
* - 0x2C
- \_\_le16
- bg\_free\_blocks\_count\_hi
- Upper 16-bits of free block count.
* - 0x2E
- \_\_le16
- bg\_free\_inodes\_count\_hi
- Upper 16-bits of free inode count.
* - 0x30
- \_\_le16
- bg\_used\_dirs\_count\_hi
- Upper 16-bits of directory count.
* - 0x32
- \_\_le16
- bg\_itable\_unused\_hi
- Upper 16-bits of unused inode count.
* - 0x34
- \_\_le32
- bg\_exclude\_bitmap\_hi
- Upper 32-bits of location of snapshot exclusion bitmap.
* - 0x38
- \_\_le16
- bg\_block\_bitmap\_csum\_hi
- Upper 16-bits of the block bitmap checksum.
* - 0x3A
- \_\_le16
- bg\_inode\_bitmap\_csum\_hi
- Upper 16-bits of the inode bitmap checksum.
* - 0x3C
- \_\_u32
- bg\_reserved
- Padding to 64 bytes.
.. _bgflags:
Block group flags can be any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- inode table and bitmap are not initialized (EXT4\_BG\_INODE\_UNINIT).
* - 0x2
- block bitmap is not initialized (EXT4\_BG\_BLOCK\_UNINIT).
* - 0x4
- inode table is zeroed (EXT4\_BG\_INODE\_ZEROED).

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.. SPDX-License-Identifier: GPL-2.0
The Contents of inode.i\_block
------------------------------
Depending on the type of file an inode describes, the 60 bytes of
storage in ``inode.i_block`` can be used in different ways. In general,
regular files and directories will use it for file block indexing
information, and special files will use it for special purposes.
Symbolic Links
~~~~~~~~~~~~~~
The target of a symbolic link will be stored in this field if the target
string is less than 60 bytes long. Otherwise, either extents or block
maps will be used to allocate data blocks to store the link target.
Direct/Indirect Block Addressing
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In ext2/3, file block numbers were mapped to logical block numbers by
means of an (up to) three level 1-1 block map. To find the logical block
that stores a particular file block, the code would navigate through
this increasingly complicated structure. Notice that there is neither a
magic number nor a checksum to provide any level of confidence that the
block isn't full of garbage.
.. ifconfig:: builder != 'latex'
.. include:: blockmap.rst
.. ifconfig:: builder == 'latex'
[Table omitted because LaTeX doesn't support nested tables.]
Note that with this block mapping scheme, it is necessary to fill out a
lot of mapping data even for a large contiguous file! This inefficiency
led to the creation of the extent mapping scheme, discussed below.
Notice also that a file using this mapping scheme cannot be placed
higher than 2^32 blocks.
Extent Tree
~~~~~~~~~~~
In ext4, the file to logical block map has been replaced with an extent
tree. Under the old scheme, allocating a contiguous run of 1,000 blocks
requires an indirect block to map all 1,000 entries; with extents, the
mapping is reduced to a single ``struct ext4_extent`` with
``ee_len = 1000``. If flex\_bg is enabled, it is possible to allocate
very large files with a single extent, at a considerable reduction in
metadata block use, and some improvement in disk efficiency. The inode
must have the extents flag (0x80000) flag set for this feature to be in
use.
Extents are arranged as a tree. Each node of the tree begins with a
``struct ext4_extent_header``. If the node is an interior node
(``eh.eh_depth`` > 0), the header is followed by ``eh.eh_entries``
instances of ``struct ext4_extent_idx``; each of these index entries
points to a block containing more nodes in the extent tree. If the node
is a leaf node (``eh.eh_depth == 0``), then the header is followed by
``eh.eh_entries`` instances of ``struct ext4_extent``; these instances
point to the file's data blocks. The root node of the extent tree is
stored in ``inode.i_block``, which allows for the first four extents to
be recorded without the use of extra metadata blocks.
The extent tree header is recorded in ``struct ext4_extent_header``,
which is 12 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le16
- eh\_magic
- Magic number, 0xF30A.
* - 0x2
- \_\_le16
- eh\_entries
- Number of valid entries following the header.
* - 0x4
- \_\_le16
- eh\_max
- Maximum number of entries that could follow the header.
* - 0x6
- \_\_le16
- eh\_depth
- Depth of this extent node in the extent tree. 0 = this extent node
points to data blocks; otherwise, this extent node points to other
extent nodes. The extent tree can be at most 5 levels deep: a logical
block number can be at most ``2^32``, and the smallest ``n`` that
satisfies ``4*(((blocksize - 12)/12)^n) >= 2^32`` is 5.
* - 0x8
- \_\_le32
- eh\_generation
- Generation of the tree. (Used by Lustre, but not standard ext4).
Internal nodes of the extent tree, also known as index nodes, are
recorded as ``struct ext4_extent_idx``, and are 12 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- ei\_block
- This index node covers file blocks from 'block' onward.
* - 0x4
- \_\_le32
- ei\_leaf\_lo
- Lower 32-bits of the block number of the extent node that is the next
level lower in the tree. The tree node pointed to can be either another
internal node or a leaf node, described below.
* - 0x8
- \_\_le16
- ei\_leaf\_hi
- Upper 16-bits of the previous field.
* - 0xA
- \_\_u16
- ei\_unused
-
Leaf nodes of the extent tree are recorded as ``struct ext4_extent``,
and are also 12 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- ee\_block
- First file block number that this extent covers.
* - 0x4
- \_\_le16
- ee\_len
- Number of blocks covered by extent. If the value of this field is <=
32768, the extent is initialized. If the value of the field is > 32768,
the extent is uninitialized and the actual extent length is ``ee_len`` -
32768. Therefore, the maximum length of a initialized extent is 32768
blocks, and the maximum length of an uninitialized extent is 32767.
* - 0x6
- \_\_le16
- ee\_start\_hi
- Upper 16-bits of the block number to which this extent points.
* - 0x8
- \_\_le32
- ee\_start\_lo
- Lower 32-bits of the block number to which this extent points.
Prior to the introduction of metadata checksums, the extent header +
extent entries always left at least 4 bytes of unallocated space at the
end of each extent tree data block (because (2^x % 12) >= 4). Therefore,
the 32-bit checksum is inserted into this space. The 4 extents in the
inode do not need checksumming, since the inode is already checksummed.
The checksum is calculated against the FS UUID, the inode number, the
inode generation, and the entire extent block leading up to (but not
including) the checksum itself.
``struct ext4_extent_tail`` is 4 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- eb\_checksum
- Checksum of the extent block, crc32c(uuid+inum+igeneration+extentblock)
Inline Data
~~~~~~~~~~~
If the inline data feature is enabled for the filesystem and the flag is
set for the inode, it is possible that the first 60 bytes of the file
data are stored here.

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.. SPDX-License-Identifier: GPL-2.0
==============================
Data Structures and Algorithms
==============================
.. include:: about.rst
.. include:: overview.rst
.. include:: globals.rst
.. include:: dynamic.rst

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.. SPDX-License-Identifier: GPL-2.0
Inline Data
-----------
The inline data feature was designed to handle the case that a file's
data is so tiny that it readily fits inside the inode, which
(theoretically) reduces disk block consumption and reduces seeks. If the
file is smaller than 60 bytes, then the data are stored inline in
``inode.i_block``. If the rest of the file would fit inside the extended
attribute space, then it might be found as an extended attribute
“system.data” within the inode body (“ibody EA”). This of course
constrains the amount of extended attributes one can attach to an inode.
If the data size increases beyond i\_block + ibody EA, a regular block
is allocated and the contents moved to that block.
Pending a change to compact the extended attribute key used to store
inline data, one ought to be able to store 160 bytes of data in a
256-byte inode (as of June 2015, when i\_extra\_isize is 28). Prior to
that, the limit was 156 bytes due to inefficient use of inode space.
The inline data feature requires the presence of an extended attribute
for “system.data”, even if the attribute value is zero length.
Inline Directories
~~~~~~~~~~~~~~~~~~
The first four bytes of i\_block are the inode number of the parent
directory. Following that is a 56-byte space for an array of directory
entries; see ``struct ext4_dir_entry``. If there is a “system.data”
attribute in the inode body, the EA value is an array of
``struct ext4_dir_entry`` as well. Note that for inline directories, the
i\_block and EA space are treated as separate dirent blocks; directory
entries cannot span the two.
Inline directory entries are not checksummed, as the inode checksum
should protect all inline data contents.

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.. SPDX-License-Identifier: GPL-2.0
Index Nodes
-----------
In a regular UNIX filesystem, the inode stores all the metadata
pertaining to the file (time stamps, block maps, extended attributes,
etc), not the directory entry. To find the information associated with a
file, one must traverse the directory files to find the directory entry
associated with a file, then load the inode to find the metadata for
that file. ext4 appears to cheat (for performance reasons) a little bit
by storing a copy of the file type (normally stored in the inode) in the
directory entry. (Compare all this to FAT, which stores all the file
information directly in the directory entry, but does not support hard
links and is in general more seek-happy than ext4 due to its simpler
block allocator and extensive use of linked lists.)
The inode table is a linear array of ``struct ext4_inode``. The table is
sized to have enough blocks to store at least
``sb.s_inode_size * sb.s_inodes_per_group`` bytes. The number of the
block group containing an inode can be calculated as
``(inode_number - 1) / sb.s_inodes_per_group``, and the offset into the
group's table is ``(inode_number - 1) % sb.s_inodes_per_group``. There
is no inode 0.
The inode checksum is calculated against the FS UUID, the inode number,
and the inode structure itself.
The inode table entry is laid out in ``struct ext4_inode``.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le16
- i\_mode
- File mode. See the table i_mode_ below.
* - 0x2
- \_\_le16
- i\_uid
- Lower 16-bits of Owner UID.
* - 0x4
- \_\_le32
- i\_size\_lo
- Lower 32-bits of size in bytes.
* - 0x8
- \_\_le32
- i\_atime
- Last access time, in seconds since the epoch. However, if the EA\_INODE
inode flag is set, this inode stores an extended attribute value and
this field contains the checksum of the value.
* - 0xC
- \_\_le32
- i\_ctime
- Last inode change time, in seconds since the epoch. However, if the
EA\_INODE inode flag is set, this inode stores an extended attribute
value and this field contains the lower 32 bits of the attribute value's
reference count.
* - 0x10
- \_\_le32
- i\_mtime
- Last data modification time, in seconds since the epoch. However, if the
EA\_INODE inode flag is set, this inode stores an extended attribute
value and this field contains the number of the inode that owns the
extended attribute.
* - 0x14
- \_\_le32
- i\_dtime
- Deletion Time, in seconds since the epoch.
* - 0x18
- \_\_le16
- i\_gid
- Lower 16-bits of GID.
* - 0x1A
- \_\_le16
- i\_links\_count
- Hard link count. Normally, ext4 does not permit an inode to have more
than 65,000 hard links. This applies to files as well as directories,
which means that there cannot be more than 64,998 subdirectories in a
directory (each subdirectory's '..' entry counts as a hard link, as does
the '.' entry in the directory itself). With the DIR\_NLINK feature
enabled, ext4 supports more than 64,998 subdirectories by setting this
field to 1 to indicate that the number of hard links is not known.
* - 0x1C
- \_\_le32
- i\_blocks\_lo
- Lower 32-bits of “block” count. If the huge\_file feature flag is not
set on the filesystem, the file consumes ``i_blocks_lo`` 512-byte blocks
on disk. If huge\_file is set and EXT4\_HUGE\_FILE\_FL is NOT set in
``inode.i_flags``, then the file consumes ``i_blocks_lo + (i_blocks_hi
<< 32)`` 512-byte blocks on disk. If huge\_file is set and
EXT4\_HUGE\_FILE\_FL IS set in ``inode.i_flags``, then this file
consumes (``i_blocks_lo + i_blocks_hi`` << 32) filesystem blocks on
disk.
* - 0x20
- \_\_le32
- i\_flags
- Inode flags. See the table i_flags_ below.
* - 0x24
- 4 bytes
- i\_osd1
- See the table i_osd1_ for more details.
* - 0x28
- 60 bytes
- i\_block[EXT4\_N\_BLOCKS=15]
- Block map or extent tree. See the section “The Contents of inode.i\_block”.
* - 0x64
- \_\_le32
- i\_generation
- File version (for NFS).
* - 0x68
- \_\_le32
- i\_file\_acl\_lo
- Lower 32-bits of extended attribute block. ACLs are of course one of
many possible extended attributes; I think the name of this field is a
result of the first use of extended attributes being for ACLs.
* - 0x6C
- \_\_le32
- i\_size\_high / i\_dir\_acl
- Upper 32-bits of file/directory size. In ext2/3 this field was named
i\_dir\_acl, though it was usually set to zero and never used.
* - 0x70
- \_\_le32
- i\_obso\_faddr
- (Obsolete) fragment address.
* - 0x74
- 12 bytes
- i\_osd2
- See the table i_osd2_ for more details.
* - 0x80
- \_\_le16
- i\_extra\_isize
- Size of this inode - 128. Alternately, the size of the extended inode
fields beyond the original ext2 inode, including this field.
* - 0x82
- \_\_le16
- i\_checksum\_hi
- Upper 16-bits of the inode checksum.
* - 0x84
- \_\_le32
- i\_ctime\_extra
- Extra change time bits. This provides sub-second precision. See Inode
Timestamps section.
* - 0x88
- \_\_le32
- i\_mtime\_extra
- Extra modification time bits. This provides sub-second precision.
* - 0x8C
- \_\_le32
- i\_atime\_extra
- Extra access time bits. This provides sub-second precision.
* - 0x90
- \_\_le32
- i\_crtime
- File creation time, in seconds since the epoch.
* - 0x94
- \_\_le32
- i\_crtime\_extra
- Extra file creation time bits. This provides sub-second precision.
* - 0x98
- \_\_le32
- i\_version\_hi
- Upper 32-bits for version number.
* - 0x9C
- \_\_le32
- i\_projid
- Project ID.
.. _i_mode:
The ``i_mode`` value is a combination of the following flags:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- S\_IXOTH (Others may execute)
* - 0x2
- S\_IWOTH (Others may write)
* - 0x4
- S\_IROTH (Others may read)
* - 0x8
- S\_IXGRP (Group members may execute)
* - 0x10
- S\_IWGRP (Group members may write)
* - 0x20
- S\_IRGRP (Group members may read)
* - 0x40
- S\_IXUSR (Owner may execute)
* - 0x80
- S\_IWUSR (Owner may write)
* - 0x100
- S\_IRUSR (Owner may read)
* - 0x200
- S\_ISVTX (Sticky bit)
* - 0x400
- S\_ISGID (Set GID)
* - 0x800
- S\_ISUID (Set UID)
* -
- These are mutually-exclusive file types:
* - 0x1000
- S\_IFIFO (FIFO)
* - 0x2000
- S\_IFCHR (Character device)
* - 0x4000
- S\_IFDIR (Directory)
* - 0x6000
- S\_IFBLK (Block device)
* - 0x8000
- S\_IFREG (Regular file)
* - 0xA000
- S\_IFLNK (Symbolic link)
* - 0xC000
- S\_IFSOCK (Socket)
.. _i_flags:
The ``i_flags`` field is a combination of these values:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- This file requires secure deletion (EXT4\_SECRM\_FL). (not implemented)
* - 0x2
- This file should be preserved, should undeletion be desired
(EXT4\_UNRM\_FL). (not implemented)
* - 0x4
- File is compressed (EXT4\_COMPR\_FL). (not really implemented)
* - 0x8
- All writes to the file must be synchronous (EXT4\_SYNC\_FL).
* - 0x10
- File is immutable (EXT4\_IMMUTABLE\_FL).
* - 0x20
- File can only be appended (EXT4\_APPEND\_FL).
* - 0x40
- The dump(1) utility should not dump this file (EXT4\_NODUMP\_FL).
* - 0x80
- Do not update access time (EXT4\_NOATIME\_FL).
* - 0x100
- Dirty compressed file (EXT4\_DIRTY\_FL). (not used)
* - 0x200
- File has one or more compressed clusters (EXT4\_COMPRBLK\_FL). (not used)
* - 0x400
- Do not compress file (EXT4\_NOCOMPR\_FL). (not used)
* - 0x800
- Encrypted inode (EXT4\_ENCRYPT\_FL). This bit value previously was
EXT4\_ECOMPR\_FL (compression error), which was never used.
* - 0x1000
- Directory has hashed indexes (EXT4\_INDEX\_FL).
* - 0x2000
- AFS magic directory (EXT4\_IMAGIC\_FL).
* - 0x4000
- File data must always be written through the journal
(EXT4\_JOURNAL\_DATA\_FL).
* - 0x8000
- File tail should not be merged (EXT4\_NOTAIL\_FL). (not used by ext4)
* - 0x10000
- All directory entry data should be written synchronously (see
``dirsync``) (EXT4\_DIRSYNC\_FL).
* - 0x20000
- Top of directory hierarchy (EXT4\_TOPDIR\_FL).
* - 0x40000
- This is a huge file (EXT4\_HUGE\_FILE\_FL).
* - 0x80000
- Inode uses extents (EXT4\_EXTENTS\_FL).
* - 0x200000
- Inode stores a large extended attribute value in its data blocks
(EXT4\_EA\_INODE\_FL).
* - 0x400000
- This file has blocks allocated past EOF (EXT4\_EOFBLOCKS\_FL).
(deprecated)
* - 0x01000000
- Inode is a snapshot (``EXT4_SNAPFILE_FL``). (not in mainline)
* - 0x04000000
- Snapshot is being deleted (``EXT4_SNAPFILE_DELETED_FL``). (not in
mainline)
* - 0x08000000
- Snapshot shrink has completed (``EXT4_SNAPFILE_SHRUNK_FL``). (not in
mainline)
* - 0x10000000
- Inode has inline data (EXT4\_INLINE\_DATA\_FL).
* - 0x20000000
- Create children with the same project ID (EXT4\_PROJINHERIT\_FL).
* - 0x80000000
- Reserved for ext4 library (EXT4\_RESERVED\_FL).
* -
- Aggregate flags:
* - 0x4BDFFF
- User-visible flags.
* - 0x4B80FF
- User-modifiable flags. Note that while EXT4\_JOURNAL\_DATA\_FL and
EXT4\_EXTENTS\_FL can be set with setattr, they are not in the kernel's
EXT4\_FL\_USER\_MODIFIABLE mask, since it needs to handle the setting of
these flags in a special manner and they are masked out of the set of
flags that are saved directly to i\_flags.
.. _i_osd1:
The ``osd1`` field has multiple meanings depending on the creator:
Linux:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- l\_i\_version
- Inode version. However, if the EA\_INODE inode flag is set, this inode
stores an extended attribute value and this field contains the upper 32
bits of the attribute value's reference count.
Hurd:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- h\_i\_translator
- ??
Masix:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- m\_i\_reserved
- ??
.. _i_osd2:
The ``osd2`` field has multiple meanings depending on the filesystem creator:
Linux:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le16
- l\_i\_blocks\_high
- Upper 16-bits of the block count. Please see the note attached to
i\_blocks\_lo.
* - 0x2
- \_\_le16
- l\_i\_file\_acl\_high
- Upper 16-bits of the extended attribute block (historically, the file
ACL location). See the Extended Attributes section below.
* - 0x4
- \_\_le16
- l\_i\_uid\_high
- Upper 16-bits of the Owner UID.
* - 0x6
- \_\_le16
- l\_i\_gid\_high
- Upper 16-bits of the GID.
* - 0x8
- \_\_le16
- l\_i\_checksum\_lo
- Lower 16-bits of the inode checksum.
* - 0xA
- \_\_le16
- l\_i\_reserved
- Unused.
Hurd:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le16
- h\_i\_reserved1
- ??
* - 0x2
- \_\_u16
- h\_i\_mode\_high
- Upper 16-bits of the file mode.
* - 0x4
- \_\_le16
- h\_i\_uid\_high
- Upper 16-bits of the Owner UID.
* - 0x6
- \_\_le16
- h\_i\_gid\_high
- Upper 16-bits of the GID.
* - 0x8
- \_\_u32
- h\_i\_author
- Author code?
Masix:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le16
- h\_i\_reserved1
- ??
* - 0x2
- \_\_u16
- m\_i\_file\_acl\_high
- Upper 16-bits of the extended attribute block (historically, the file
ACL location).
* - 0x4
- \_\_u32
- m\_i\_reserved2[2]
- ??
Inode Size
~~~~~~~~~~
In ext2 and ext3, the inode structure size was fixed at 128 bytes
(``EXT2_GOOD_OLD_INODE_SIZE``) and each inode had a disk record size of
128 bytes. Starting with ext4, it is possible to allocate a larger
on-disk inode at format time for all inodes in the filesystem to provide
space beyond the end of the original ext2 inode. The on-disk inode
record size is recorded in the superblock as ``s_inode_size``. The
number of bytes actually used by struct ext4\_inode beyond the original
128-byte ext2 inode is recorded in the ``i_extra_isize`` field for each
inode, which allows struct ext4\_inode to grow for a new kernel without
having to upgrade all of the on-disk inodes. Access to fields beyond
EXT2\_GOOD\_OLD\_INODE\_SIZE should be verified to be within
``i_extra_isize``. By default, ext4 inode records are 256 bytes, and (as
of October 2013) the inode structure is 156 bytes
(``i_extra_isize = 28``). The extra space between the end of the inode
structure and the end of the inode record can be used to store extended
attributes. Each inode record can be as large as the filesystem block
size, though this is not terribly efficient.
Finding an Inode
~~~~~~~~~~~~~~~~
Each block group contains ``sb->s_inodes_per_group`` inodes. Because
inode 0 is defined not to exist, this formula can be used to find the
block group that an inode lives in:
``bg = (inode_num - 1) / sb->s_inodes_per_group``. The particular inode
can be found within the block group's inode table at
``index = (inode_num - 1) % sb->s_inodes_per_group``. To get the byte
address within the inode table, use
``offset = index * sb->s_inode_size``.
Inode Timestamps
~~~~~~~~~~~~~~~~
Four timestamps are recorded in the lower 128 bytes of the inode
structure -- inode change time (ctime), access time (atime), data
modification time (mtime), and deletion time (dtime). The four fields
are 32-bit signed integers that represent seconds since the Unix epoch
(1970-01-01 00:00:00 GMT), which means that the fields will overflow in
January 2038. For inodes that are not linked from any directory but are
still open (orphan inodes), the dtime field is overloaded for use with
the orphan list. The superblock field ``s_last_orphan`` points to the
first inode in the orphan list; dtime is then the number of the next
orphaned inode, or zero if there are no more orphans.
If the inode structure size ``sb->s_inode_size`` is larger than 128
bytes and the ``i_inode_extra`` field is large enough to encompass the
respective ``i_[cma]time_extra`` field, the ctime, atime, and mtime
inode fields are widened to 64 bits. Within this “extra” 32-bit field,
the lower two bits are used to extend the 32-bit seconds field to be 34
bit wide; the upper 30 bits are used to provide nanosecond timestamp
accuracy. Therefore, timestamps should not overflow until May 2446.
dtime was not widened. There is also a fifth timestamp to record inode
creation time (crtime); this field is 64-bits wide and decoded in the
same manner as 64-bit [cma]time. Neither crtime nor dtime are accessible
through the regular stat() interface, though debugfs will report them.
We use the 32-bit signed time value plus (2^32 \* (extra epoch bits)).
In other words:
.. list-table::
:widths: 20 20 20 20 20
:header-rows: 1
* - Extra epoch bits
- MSB of 32-bit time
- Adjustment for signed 32-bit to 64-bit tv\_sec
- Decoded 64-bit tv\_sec
- valid time range
* - 0 0
- 1
- 0
- ``-0x80000000 - -0x00000001``
- 1901-12-13 to 1969-12-31
* - 0 0
- 0
- 0
- ``0x000000000 - 0x07fffffff``
- 1970-01-01 to 2038-01-19
* - 0 1
- 1
- 0x100000000
- ``0x080000000 - 0x0ffffffff``
- 2038-01-19 to 2106-02-07
* - 0 1
- 0
- 0x100000000
- ``0x100000000 - 0x17fffffff``
- 2106-02-07 to 2174-02-25
* - 1 0
- 1
- 0x200000000
- ``0x180000000 - 0x1ffffffff``
- 2174-02-25 to 2242-03-16
* - 1 0
- 0
- 0x200000000
- ``0x200000000 - 0x27fffffff``
- 2242-03-16 to 2310-04-04
* - 1 1
- 1
- 0x300000000
- ``0x280000000 - 0x2ffffffff``
- 2310-04-04 to 2378-04-22
* - 1 1
- 0
- 0x300000000
- ``0x300000000 - 0x37fffffff``
- 2378-04-22 to 2446-05-10
This is a somewhat odd encoding since there are effectively seven times
as many positive values as negative values. There have also been
long-standing bugs decoding and encoding dates beyond 2038, which don't
seem to be fixed as of kernel 3.12 and e2fsprogs 1.42.8. 64-bit kernels
incorrectly use the extra epoch bits 1,1 for dates between 1901 and
1970. At some point the kernel will be fixed and e2fsck will fix this
situation, assuming that it is run before 2310.

611
Documentation/filesystems/ext4/ondisk/journal.rst Normal file
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@ -0,0 +1,611 @@
.. SPDX-License-Identifier: GPL-2.0
Journal (jbd2)
--------------
Introduced in ext3, the ext4 filesystem employs a journal to protect the
filesystem against corruption in the case of a system crash. A small
continuous region of disk (default 128MiB) is reserved inside the
filesystem as a place to land “important” data writes on-disk as quickly
as possible. Once the important data transaction is fully written to the
disk and flushed from the disk write cache, a record of the data being
committed is also written to the journal. At some later point in time,
the journal code writes the transactions to their final locations on
disk (this could involve a lot of seeking or a lot of small
read-write-erases) before erasing the commit record. Should the system
crash during the second slow write, the journal can be replayed all the
way to the latest commit record, guaranteeing the atomicity of whatever
gets written through the journal to the disk. The effect of this is to
guarantee that the filesystem does not become stuck midway through a
metadata update.
For performance reasons, ext4 by default only writes filesystem metadata
through the journal. This means that file data blocks are /not/
guaranteed to be in any consistent state after a crash. If this default
guarantee level (``data=ordered``) is not satisfactory, there is a mount
option to control journal behavior. If ``data=journal``, all data and
metadata are written to disk through the journal. This is slower but
safest. If ``data=writeback``, dirty data blocks are not flushed to the
disk before the metadata are written to disk through the journal.
The journal inode is typically inode 8. The first 68 bytes of the
journal inode are replicated in the ext4 superblock. The journal itself
is normal (but hidden) file within the filesystem. The file usually
consumes an entire block group, though mke2fs tries to put it in the
middle of the disk.
All fields in jbd2 are written to disk in big-endian order. This is the
opposite of ext4.
NOTE: Both ext4 and ocfs2 use jbd2.
The maximum size of a journal embedded in an ext4 filesystem is 2^32
blocks. jbd2 itself does not seem to care.
Layout
~~~~~~
Generally speaking, the journal has this format:
.. list-table::
:widths: 1 1 78
:header-rows: 1
* - Superblock
- descriptor\_block (data\_blocks or revocation\_block) [more data or
revocations] commmit\_block
- [more transactions...]
* -
- One transaction
-
Notice that a transaction begins with either a descriptor and some data,
or a block revocation list. A finished transaction always ends with a
commit. If there is no commit record (or the checksums don't match), the
transaction will be discarded during replay.
External Journal
~~~~~~~~~~~~~~~~
Optionally, an ext4 filesystem can be created with an external journal
device (as opposed to an internal journal, which uses a reserved inode).
In this case, on the filesystem device, ``s_journal_inum`` should be
zero and ``s_journal_uuid`` should be set. On the journal device there
will be an ext4 super block in the usual place, with a matching UUID.
The journal superblock will be in the next full block after the
superblock.
.. list-table::
:widths: 1 1 1 1 76
:header-rows: 1
* - 1024 bytes of padding
- ext4 Superblock
- Journal Superblock
- descriptor\_block (data\_blocks or revocation\_block) [more data or
revocations] commmit\_block
- [more transactions...]
* -
-
-
- One transaction
-
Block Header
~~~~~~~~~~~~
Every block in the journal starts with a common 12-byte header
``struct journal_header_s``:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_be32
- h\_magic
- jbd2 magic number, 0xC03B3998.
* - 0x4
- \_\_be32
- h\_blocktype
- Description of what this block contains. See the jbd2_blocktype_ table
below.
* - 0x8
- \_\_be32
- h\_sequence
- The transaction ID that goes with this block.
.. _jbd2_blocktype:
The journal block type can be any one of:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 1
- Descriptor. This block precedes a series of data blocks that were
written through the journal during a transaction.
* - 2
- Block commit record. This block signifies the completion of a
transaction.
* - 3
- Journal superblock, v1.
* - 4
- Journal superblock, v2.
* - 5
- Block revocation records. This speeds up recovery by enabling the
journal to skip writing blocks that were subsequently rewritten.
Super Block
~~~~~~~~~~~
The super block for the journal is much simpler as compared to ext4's.
The key data kept within are size of the journal, and where to find the
start of the log of transactions.
The journal superblock is recorded as ``struct journal_superblock_s``,
which is 1024 bytes long:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* -
-
-
- Static information describing the journal.
* - 0x0
- journal\_header\_t (12 bytes)
- s\_header
- Common header identifying this as a superblock.
* - 0xC
- \_\_be32
- s\_blocksize
- Journal device block size.
* - 0x10
- \_\_be32
- s\_maxlen
- Total number of blocks in this journal.
* - 0x14
- \_\_be32
- s\_first
- First block of log information.
* -
-
-
- Dynamic information describing the current state of the log.
* - 0x18
- \_\_be32
- s\_sequence
- First commit ID expected in log.
* - 0x1C
- \_\_be32
- s\_start
- Block number of the start of log. Contrary to the comments, this field
being zero does not imply that the journal is clean!
* - 0x20
- \_\_be32
- s\_errno
- Error value, as set by jbd2\_journal\_abort().
* -
-
-
- The remaining fields are only valid in a v2 superblock.
* - 0x24
- \_\_be32
- s\_feature\_compat;
- Compatible feature set. See the table jbd2_compat_ below.
* - 0x28
- \_\_be32
- s\_feature\_incompat
- Incompatible feature set. See the table jbd2_incompat_ below.
* - 0x2C
- \_\_be32
- s\_feature\_ro\_compat
- Read-only compatible feature set. There aren't any of these currently.
* - 0x30
- \_\_u8
- s\_uuid[16]
- 128-bit uuid for journal. This is compared against the copy in the ext4
super block at mount time.
* - 0x40
- \_\_be32
- s\_nr\_users
- Number of file systems sharing this journal.
* - 0x44
- \_\_be32
- s\_dynsuper
- Location of dynamic super block copy. (Not used?)
* - 0x48
- \_\_be32
- s\_max\_transaction
- Limit of journal blocks per transaction. (Not used?)
* - 0x4C
- \_\_be32
- s\_max\_trans\_data
- Limit of data blocks per transaction. (Not used?)
* - 0x50
- \_\_u8
- s\_checksum\_type
- Checksum algorithm used for the journal. See jbd2_checksum_type_ for
more info.
* - 0x51
- \_\_u8[3]
- s\_padding2
-
* - 0x54
- \_\_u32
- s\_padding[42]
-
* - 0xFC
- \_\_be32
- s\_checksum
- Checksum of the entire superblock, with this field set to zero.
* - 0x100
- \_\_u8
- s\_users[16\*48]
- ids of all file systems sharing the log. e2fsprogs/Linux don't allow
shared external journals, but I imagine Lustre (or ocfs2?), which use
the jbd2 code, might.
.. _jbd2_compat:
The journal compat features are any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- Journal maintains checksums on the data blocks.
(JBD2\_FEATURE\_COMPAT\_CHECKSUM)
.. _jbd2_incompat:
The journal incompat features are any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- Journal has block revocation records. (JBD2\_FEATURE\_INCOMPAT\_REVOKE)
* - 0x2
- Journal can deal with 64-bit block numbers.
(JBD2\_FEATURE\_INCOMPAT\_64BIT)
* - 0x4
- Journal commits asynchronously. (JBD2\_FEATURE\_INCOMPAT\_ASYNC\_COMMIT)
* - 0x8
- This journal uses v2 of the checksum on-disk format. Each journal
metadata block gets its own checksum, and the block tags in the
descriptor table contain checksums for each of the data blocks in the
journal. (JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2)
* - 0x10
- This journal uses v3 of the checksum on-disk format. This is the same as
v2, but the journal block tag size is fixed regardless of the size of
block numbers. (JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3)
.. _jbd2_checksum_type:
Journal checksum type codes are one of the following. crc32 or crc32c are the
most likely choices.
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 1
- CRC32
* - 2
- MD5
* - 3
- SHA1
* - 4
- CRC32C
Descriptor Block
~~~~~~~~~~~~~~~~
The descriptor block contains an array of journal block tags that
describe the final locations of the data blocks that follow in the
journal. Descriptor blocks are open-coded instead of being completely
described by a data structure, but here is the block structure anyway.
Descriptor blocks consume at least 36 bytes, but use a full block:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Descriptor
* - 0x0
- journal\_header\_t
- (open coded)
- Common block header.
* - 0xC
- struct journal\_block\_tag\_s
- open coded array[]
- Enough tags either to fill up the block or to describe all the data
blocks that follow this descriptor block.
Journal block tags have any of the following formats, depending on which
journal feature and block tag flags are set.
If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 is set, the journal block tag is
defined as ``struct journal_block_tag3_s``, which looks like the
following. The size is 16 or 32 bytes.
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Descriptor
* - 0x0
- \_\_be32
- t\_blocknr
- Lower 32-bits of the location of where the corresponding data block
should end up on disk.
* - 0x4
- \_\_be32
- t\_flags
- Flags that go with the descriptor. See the table jbd2_tag_flags_ for
more info.
* - 0x8
- \_\_be32
- t\_blocknr\_high
- Upper 32-bits of the location of where the corresponding data block
should end up on disk. This is zero if JBD2\_FEATURE\_INCOMPAT\_64BIT is
not enabled.
* - 0xC
- \_\_be32
- t\_checksum
- Checksum of the journal UUID, the sequence number, and the data block.
* -
-
-
- This field appears to be open coded. It always comes at the end of the
tag, after t_checksum. This field is not present if the "same UUID" flag
is set.
* - 0x8 or 0xC
- char
- uuid[16]
- A UUID to go with this tag. This field appears to be copied from the
``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
field.
.. _jbd2_tag_flags:
The journal tag flags are any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- On-disk block is escaped. The first four bytes of the data block just
happened to match the jbd2 magic number.
* - 0x2
- This block has the same UUID as previous, therefore the UUID field is
omitted.
* - 0x4
- The data block was deleted by the transaction. (Not used?)
* - 0x8
- This is the last tag in this descriptor block.
If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 is NOT set, the journal block tag
is defined as ``struct journal_block_tag_s``, which looks like the
following. The size is 8, 12, 24, or 28 bytes:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Descriptor
* - 0x0
- \_\_be32
- t\_blocknr
- Lower 32-bits of the location of where the corresponding data block
should end up on disk.
* - 0x4
- \_\_be16
- t\_checksum
- Checksum of the journal UUID, the sequence number, and the data block.
Note that only the lower 16 bits are stored.
* - 0x6
- \_\_be16
- t\_flags
- Flags that go with the descriptor. See the table jbd2_tag_flags_ for
more info.
* -
-
-
- This next field is only present if the super block indicates support for
64-bit block numbers.
* - 0x8
- \_\_be32
- t\_blocknr\_high
- Upper 32-bits of the location of where the corresponding data block
should end up on disk.
* -
-
-
- This field appears to be open coded. It always comes at the end of the
tag, after t_flags or t_blocknr_high. This field is not present if the
"same UUID" flag is set.
* - 0x8 or 0xC
- char
- uuid[16]
- A UUID to go with this tag. This field appears to be copied from the
``j_uuid`` field in ``struct journal_s``, but only tune2fs touches that
field.
If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or
JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 are set, the end of the block is a
``struct jbd2_journal_block_tail``, which looks like this:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Descriptor
* - 0x0
- \_\_be32
- t\_checksum
- Checksum of the journal UUID + the descriptor block, with this field set
to zero.
Data Block
~~~~~~~~~~
In general, the data blocks being written to disk through the journal
are written verbatim into the journal file after the descriptor block.
However, if the first four bytes of the block match the jbd2 magic
number then those four bytes are replaced with zeroes and the “escaped”
flag is set in the descriptor block tag.
Revocation Block
~~~~~~~~~~~~~~~~
A revocation block is used to prevent replay of a block in an earlier
transaction. This is used to mark blocks that were journalled at one
time but are no longer journalled. Typically this happens if a metadata
block is freed and re-allocated as a file data block; in this case, a
journal replay after the file block was written to disk will cause
corruption.
**NOTE**: This mechanism is NOT used to express “this journal block is
superseded by this other journal block”, as the author (djwong)
mistakenly thought. Any block being added to a transaction will cause
the removal of all existing revocation records for that block.
Revocation blocks are described in
``struct jbd2_journal_revoke_header_s``, are at least 16 bytes in
length, but use a full block:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- journal\_header\_t
- r\_header
- Common block header.
* - 0xC
- \_\_be32
- r\_count
- Number of bytes used in this block.
* - 0x10
- \_\_be32 or \_\_be64
- blocks[0]
- Blocks to revoke.
After r\_count is a linear array of block numbers that are effectively
revoked by this transaction. The size of each block number is 8 bytes if
the superblock advertises 64-bit block number support, or 4 bytes
otherwise.
If JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or
JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3 are set, the end of the revocation
block is a ``struct jbd2_journal_revoke_tail``, which has this format:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_be32
- r\_checksum
- Checksum of the journal UUID + revocation block
Commit Block
~~~~~~~~~~~~
The commit block is a sentry that indicates that a transaction has been
completely written to the journal. Once this commit block reaches the
journal, the data stored with this transaction can be written to their
final locations on disk.
The commit block is described by ``struct commit_header``, which is 32
bytes long (but uses a full block):
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Descriptor
* - 0x0
- journal\_header\_s
- (open coded)
- Common block header.
* - 0xC
- unsigned char
- h\_chksum\_type
- The type of checksum to use to verify the integrity of the data blocks
in the transaction. See jbd2_checksum_type_ for more info.
* - 0xD
- unsigned char
- h\_chksum\_size
- The number of bytes used by the checksum. Most likely 4.
* - 0xE
- unsigned char
- h\_padding[2]
-
* - 0x10
- \_\_be32
- h\_chksum[JBD2\_CHECKSUM\_BYTES]
- 32 bytes of space to store checksums. If
JBD2\_FEATURE\_INCOMPAT\_CSUM\_V2 or JBD2\_FEATURE\_INCOMPAT\_CSUM\_V3
are set, the first ``__be32`` is the checksum of the journal UUID and
the entire commit block, with this field zeroed. If
JBD2\_FEATURE\_COMPAT\_CHECKSUM is set, the first ``__be32`` is the
crc32 of all the blocks already written to the transaction.
* - 0x30
- \_\_be64
- h\_commit\_sec
- The time that the transaction was committed, in seconds since the epoch.
* - 0x38
- \_\_be32
- h\_commit\_nsec
- Nanoseconds component of the above timestamp.

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.. SPDX-License-Identifier: GPL-2.0
Multiple Mount Protection
-------------------------
Multiple mount protection (MMP) is a feature that protects the
filesystem against multiple hosts trying to use the filesystem
simultaneously. When a filesystem is opened (for mounting, or fsck,
etc.), the MMP code running on the node (call it node A) checks a
sequence number. If the sequence number is EXT4\_MMP\_SEQ\_CLEAN, the
open continues. If the sequence number is EXT4\_MMP\_SEQ\_FSCK, then
fsck is (hopefully) running, and open fails immediately. Otherwise, the
open code will wait for twice the specified MMP check interval and check
the sequence number again. If the sequence number has changed, then the
filesystem is active on another machine and the open fails. If the MMP
code passes all of those checks, a new MMP sequence number is generated
and written to the MMP block, and the mount proceeds.
While the filesystem is live, the kernel sets up a timer to re-check the
MMP block at the specified MMP check interval. To perform the re-check,
the MMP sequence number is re-read; if it does not match the in-memory
MMP sequence number, then another node (node B) has mounted the
filesystem, and node A remounts the filesystem read-only. If the
sequence numbers match, the sequence number is incremented both in
memory and on disk, and the re-check is complete.
The hostname and device filename are written into the MMP block whenever
an open operation succeeds. The MMP code does not use these values; they
are provided purely for informational purposes.
The checksum is calculated against the FS UUID and the MMP structure.
The MMP structure (``struct mmp_struct``) is as follows:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Type
- Name
- Description
* - 0x0
- \_\_le32
- mmp\_magic
- Magic number for MMP, 0x004D4D50 (“MMP”).
* - 0x4
- \_\_le32
- mmp\_seq
- Sequence number, updated periodically.
* - 0x8
- \_\_le64
- mmp\_time
- Time that the MMP block was last updated.
* - 0x10
- char[64]
- mmp\_nodename
- Hostname of the node that opened the filesystem.
* - 0x50
- char[32]
- mmp\_bdevname
- Block device name of the filesystem.
* - 0x70
- \_\_le16
- mmp\_check\_interval
- The MMP re-check interval, in seconds.
* - 0x72
- \_\_le16
- mmp\_pad1
- Zero.
* - 0x74
- \_\_le32[226]
- mmp\_pad2
- Zero.
* - 0x3FC
- \_\_le32
- mmp\_checksum
- Checksum of the MMP block.

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.. SPDX-License-Identifier: GPL-2.0
High Level Design
=================
An ext4 file system is split into a series of block groups. To reduce
performance difficulties due to fragmentation, the block allocator tries
very hard to keep each file's blocks within the same group, thereby
reducing seek times. The size of a block group is specified in
``sb.s_blocks_per_group`` blocks, though it can also calculated as 8 \*
``block_size_in_bytes``. With the default block size of 4KiB, each group
will contain 32,768 blocks, for a length of 128MiB. The number of block
groups is the size of the device divided by the size of a block group.
All fields in ext4 are written to disk in little-endian order. HOWEVER,
all fields in jbd2 (the journal) are written to disk in big-endian
order.
.. include:: blocks.rst
.. include:: blockgroup.rst
.. include:: special_inodes.rst
.. include:: allocators.rst
.. include:: checksums.rst
.. include:: bigalloc.rst
.. include:: inlinedata.rst
.. include:: eainode.rst

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.. SPDX-License-Identifier: GPL-2.0
Special inodes
--------------
ext4 reserves some inode for special features, as follows:
.. list-table::
:widths: 1 79
:header-rows: 1
* - inode Number
- Purpose
* - 0
- Doesn't exist; there is no inode 0.
* - 1
- List of defective blocks.
* - 2
- Root directory.
* - 3
- User quota.
* - 4
- Group quota.
* - 5
- Boot loader.
* - 6
- Undelete directory.
* - 7
- Reserved group descriptors inode. (“resize inode”)
* - 8
- Journal inode.
* - 9
- The “exclude” inode, for snapshots(?)
* - 10
- Replica inode, used for some non-upstream feature?
* - 11
- Traditional first non-reserved inode. Usually this is the lost+found directory. See s\_first\_ino in the superblock.

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.. SPDX-License-Identifier: GPL-2.0
Super Block
-----------
The superblock records various information about the enclosing
filesystem, such as block counts, inode counts, supported features,
maintenance information, and more.
If the sparse\_super feature flag is set, redundant copies of the
superblock and group descriptors are kept only in the groups whose group
number is either 0 or a power of 3, 5, or 7. If the flag is not set,
redundant copies are kept in all groups.
The superblock checksum is calculated against the superblock structure,
which includes the FS UUID.
The ext4 superblock is laid out as follows in
``struct ext4_super_block``:
.. list-table::
:widths: 1 1 1 77
:header-rows: 1
* - Offset
- Size
- Name
- Description
* - 0x0
- \_\_le32
- s\_inodes\_count
- Total inode count.
* - 0x4
- \_\_le32
- s\_blocks\_count\_lo
- Total block count.
* - 0x8
- \_\_le32
- s\_r\_blocks\_count\_lo
- This number of blocks can only be allocated by the super-user.
* - 0xC
- \_\_le32
- s\_free\_blocks\_count\_lo
- Free block count.
* - 0x10
- \_\_le32
- s\_free\_inodes\_count
- Free inode count.
* - 0x14
- \_\_le32
- s\_first\_data\_block
- First data block. This must be at least 1 for 1k-block filesystems and
is typically 0 for all other block sizes.
* - 0x18
- \_\_le32
- s\_log\_block\_size
- Block size is 2 ^ (10 + s\_log\_block\_size).
* - 0x1C
- \_\_le32
- s\_log\_cluster\_size
- Cluster size is (2 ^ s\_log\_cluster\_size) blocks if bigalloc is
enabled. Otherwise s\_log\_cluster\_size must equal s\_log\_block\_size.
* - 0x20
- \_\_le32
- s\_blocks\_per\_group
- Blocks per group.
* - 0x24
- \_\_le32
- s\_clusters\_per\_group
- Clusters per group, if bigalloc is enabled. Otherwise
s\_clusters\_per\_group must equal s\_blocks\_per\_group.
* - 0x28
- \_\_le32
- s\_inodes\_per\_group
- Inodes per group.
* - 0x2C
- \_\_le32
- s\_mtime
- Mount time, in seconds since the epoch.
* - 0x30
- \_\_le32
- s\_wtime
- Write time, in seconds since the epoch.
* - 0x34
- \_\_le16
- s\_mnt\_count
- Number of mounts since the last fsck.
* - 0x36
- \_\_le16
- s\_max\_mnt\_count
- Number of mounts beyond which a fsck is needed.
* - 0x38
- \_\_le16
- s\_magic
- Magic signature, 0xEF53
* - 0x3A
- \_\_le16
- s\_state
- File system state. See super_state_ for more info.
* - 0x3C
- \_\_le16
- s\_errors
- Behaviour when detecting errors. See super_errors_ for more info.
* - 0x3E
- \_\_le16
- s\_minor\_rev\_level
- Minor revision level.
* - 0x40
- \_\_le32
- s\_lastcheck
- Time of last check, in seconds since the epoch.
* - 0x44
- \_\_le32
- s\_checkinterval
- Maximum time between checks, in seconds.
* - 0x48
- \_\_le32
- s\_creator\_os
- Creator OS. See the table super_creator_ for more info.
* - 0x4C
- \_\_le32
- s\_rev\_level
- Revision level. See the table super_revision_ for more info.
* - 0x50
- \_\_le16
- s\_def\_resuid
- Default uid for reserved blocks.
* - 0x52
- \_\_le16
- s\_def\_resgid
- Default gid for reserved blocks.
* -
-
-
- These fields are for EXT4_DYNAMIC_REV superblocks only.
Note: the difference between the compatible feature set and the
incompatible feature set is that if there is a bit set in the
incompatible feature set that the kernel doesn't know about, it should
refuse to mount the filesystem.
e2fsck's requirements are more strict; if it doesn't know
about a feature in either the compatible or incompatible feature set, it
must abort and not try to meddle with things it doesn't understand...
* - 0x54
- \_\_le32
- s\_first\_ino
- First non-reserved inode.
* - 0x58
- \_\_le16
- s\_inode\_size
- Size of inode structure, in bytes.
* - 0x5A
- \_\_le16
- s\_block\_group\_nr
- Block group # of this superblock.
* - 0x5C
- \_\_le32
- s\_feature\_compat
- Compatible feature set flags. Kernel can still read/write this fs even
if it doesn't understand a flag; fsck should not do that. See the
super_compat_ table for more info.
* - 0x60
- \_\_le32
- s\_feature\_incompat
- Incompatible feature set. If the kernel or fsck doesn't understand one
of these bits, it should stop. See the super_incompat_ table for more
info.
* - 0x64
- \_\_le32
- s\_feature\_ro\_compat
- Readonly-compatible feature set. If the kernel doesn't understand one of
these bits, it can still mount read-only. See the super_rocompat_ table
for more info.
* - 0x68
- \_\_u8
- s\_uuid[16]
- 128-bit UUID for volume.
* - 0x78
- char
- s\_volume\_name[16]
- Volume label.
* - 0x88
- char
- s\_last\_mounted[64]
- Directory where filesystem was last mounted.
* - 0xC8
- \_\_le32
- s\_algorithm\_usage\_bitmap
- For compression (Not used in e2fsprogs/Linux)
* -
-
-
- Performance hints. Directory preallocation should only happen if the
EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
* - 0xCC
- \_\_u8
- s\_prealloc\_blocks
- #. of blocks to try to preallocate for ... files? (Not used in
e2fsprogs/Linux)
* - 0xCD
- \_\_u8
- s\_prealloc\_dir\_blocks
- #. of blocks to preallocate for directories. (Not used in
e2fsprogs/Linux)
* - 0xCE
- \_\_le16
- s\_reserved\_gdt\_blocks
- Number of reserved GDT entries for future filesystem expansion.
* -
-
-
- Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is
set.
* - 0xD0
- \_\_u8
- s\_journal\_uuid[16]
- UUID of journal superblock
* - 0xE0
- \_\_le32
- s\_journal\_inum
- inode number of journal file.
* - 0xE4
- \_\_le32
- s\_journal\_dev
- Device number of journal file, if the external journal feature flag is
set.
* - 0xE8
- \_\_le32
- s\_last\_orphan
- Start of list of orphaned inodes to delete.
* - 0xEC
- \_\_le32
- s\_hash\_seed[4]
- HTREE hash seed.
* - 0xFC
- \_\_u8
- s\_def\_hash\_version
- Default hash algorithm to use for directory hashes. See super_def_hash_
for more info.
* - 0xFD
- \_\_u8
- s\_jnl\_backup\_type
- If this value is 0 or EXT3\_JNL\_BACKUP\_BLOCKS (1), then the
``s_jnl_blocks`` field contains a duplicate copy of the inode's
``i_block[]`` array and ``i_size``.
* - 0xFE
- \_\_le16
- s\_desc\_size
- Size of group descriptors, in bytes, if the 64bit incompat feature flag
is set.
* - 0x100
- \_\_le32
- s\_default\_mount\_opts
- Default mount options. See the super_mountopts_ table for more info.
* - 0x104
- \_\_le32
- s\_first\_meta\_bg
- First metablock block group, if the meta\_bg feature is enabled.
* - 0x108
- \_\_le32
- s\_mkfs\_time
- When the filesystem was created, in seconds since the epoch.
* - 0x10C
- \_\_le32
- s\_jnl\_blocks[17]
- Backup copy of the journal inode's ``i_block[]`` array in the first 15
elements and i\_size\_high and i\_size in the 16th and 17th elements,
respectively.
* -
-
-
- 64bit support is valid only if EXT4_FEATURE_COMPAT_64BIT is set.
* - 0x150
- \_\_le32
- s\_blocks\_count\_hi
- High 32-bits of the block count.
* - 0x154
- \_\_le32
- s\_r\_blocks\_count\_hi
- High 32-bits of the reserved block count.
* - 0x158
- \_\_le32
- s\_free\_blocks\_count\_hi
- High 32-bits of the free block count.
* - 0x15C
- \_\_le16
- s\_min\_extra\_isize
- All inodes have at least # bytes.
* - 0x15E
- \_\_le16
- s\_want\_extra\_isize
- New inodes should reserve # bytes.
* - 0x160
- \_\_le32
- s\_flags
- Miscellaneous flags. See the super_flags_ table for more info.
* - 0x164
- \_\_le16
- s\_raid\_stride
- RAID stride. This is the number of logical blocks read from or written
to the disk before moving to the next disk. This affects the placement
of filesystem metadata, which will hopefully make RAID storage faster.
* - 0x166
- \_\_le16
- s\_mmp\_interval
- #. seconds to wait in multi-mount prevention (MMP) checking. In theory,
MMP is a mechanism to record in the superblock which host and device
have mounted the filesystem, in order to prevent multiple mounts. This
feature does not seem to be implemented...
* - 0x168
- \_\_le64
- s\_mmp\_block
- Block # for multi-mount protection data.
* - 0x170
- \_\_le32
- s\_raid\_stripe\_width
- RAID stripe width. This is the number of logical blocks read from or
written to the disk before coming back to the current disk. This is used
by the block allocator to try to reduce the number of read-modify-write
operations in a RAID5/6.
* - 0x174
- \_\_u8
- s\_log\_groups\_per\_flex
- Size of a flexible block group is 2 ^ ``s_log_groups_per_flex``.
* - 0x175
- \_\_u8
- s\_checksum\_type
- Metadata checksum algorithm type. The only valid value is 1 (crc32c).
* - 0x176
- \_\_le16
- s\_reserved\_pad
-
* - 0x178
- \_\_le64
- s\_kbytes\_written
- Number of KiB written to this filesystem over its lifetime.
* - 0x180
- \_\_le32
- s\_snapshot\_inum
- inode number of active snapshot. (Not used in e2fsprogs/Linux.)
* - 0x184
- \_\_le32
- s\_snapshot\_id
- Sequential ID of active snapshot. (Not used in e2fsprogs/Linux.)
* - 0x188
- \_\_le64
- s\_snapshot\_r\_blocks\_count
- Number of blocks reserved for active snapshot's future use. (Not used in
e2fsprogs/Linux.)
* - 0x190
- \_\_le32
- s\_snapshot\_list
- inode number of the head of the on-disk snapshot list. (Not used in
e2fsprogs/Linux.)
* - 0x194
- \_\_le32
- s\_error\_count
- Number of errors seen.
* - 0x198
- \_\_le32
- s\_first\_error\_time
- First time an error happened, in seconds since the epoch.
* - 0x19C
- \_\_le32
- s\_first\_error\_ino
- inode involved in first error.
* - 0x1A0
- \_\_le64
- s\_first\_error\_block
- Number of block involved of first error.
* - 0x1A8
- \_\_u8
- s\_first\_error\_func[32]
- Name of function where the error happened.
* - 0x1C8
- \_\_le32
- s\_first\_error\_line
- Line number where error happened.
* - 0x1CC
- \_\_le32
- s\_last\_error\_time
- Time of most recent error, in seconds since the epoch.
* - 0x1D0
- \_\_le32
- s\_last\_error\_ino
- inode involved in most recent error.
* - 0x1D4
- \_\_le32
- s\_last\_error\_line
- Line number where most recent error happened.
* - 0x1D8
- \_\_le64
- s\_last\_error\_block
- Number of block involved in most recent error.
* - 0x1E0
- \_\_u8
- s\_last\_error\_func[32]
- Name of function where the most recent error happened.
* - 0x200
- \_\_u8
- s\_mount\_opts[64]
- ASCIIZ string of mount options.
* - 0x240
- \_\_le32
- s\_usr\_quota\_inum
- Inode number of user `quota <quota>`__ file.
* - 0x244
- \_\_le32
- s\_grp\_quota\_inum
- Inode number of group `quota <quota>`__ file.
* - 0x248
- \_\_le32
- s\_overhead\_blocks
- Overhead blocks/clusters in fs. (Huh? This field is always zero, which
means that the kernel calculates it dynamically.)
* - 0x24C
- \_\_le32
- s\_backup\_bgs[2]
- Block groups containing superblock backups (if sparse\_super2)
* - 0x254
- \_\_u8
- s\_encrypt\_algos[4]
- Encryption algorithms in use. There can be up to four algorithms in use
at any time; valid algorithm codes are given in the super_encrypt_ table
below.
* - 0x258
- \_\_u8
- s\_encrypt\_pw\_salt[16]
- Salt for the string2key algorithm for encryption.
* - 0x268
- \_\_le32
- s\_lpf\_ino
- Inode number of lost+found
* - 0x26C
- \_\_le32
- s\_prj\_quota\_inum
- Inode that tracks project quotas.
* - 0x270
- \_\_le32
- s\_checksum\_seed
- Checksum seed used for metadata\_csum calculations. This value is
crc32c(~0, $orig\_fs\_uuid).
* - 0x274
- \_\_u8
- s\_wtime_hi
- Upper 8 bits of the s_wtime field.
* - 0x275
- \_\_u8
- s\_wtime_hi
- Upper 8 bits of the s_mtime field.
* - 0x276
- \_\_u8
- s\_mkfs_time_hi
- Upper 8 bits of the s_mkfs_time field.
* - 0x277
- \_\_u8
- s\_lastcheck_hi
- Upper 8 bits of the s_lastcheck_hi field.
* - 0x278
- \_\_u8
- s\_first_error_time_hi
- Upper 8 bits of the s_first_error_time_hi field.
* - 0x279
- \_\_u8
- s\_last_error_time_hi
- Upper 8 bits of the s_last_error_time_hi field.
* - 0x27A
- \_\_u8[2]
- s\_pad
- Zero padding.
* - 0x27C
- \_\_le32
- s\_reserved[96]
- Padding to the end of the block.
* - 0x3FC
- \_\_le32
- s\_checksum
- Superblock checksum.
.. _super_state:
The superblock state is some combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0001
- Cleanly umounted
* - 0x0002
- Errors detected
* - 0x0004
- Orphans being recovered
.. _super_errors:
The superblock error policy is one of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 1
- Continue
* - 2
- Remount read-only
* - 3
- Panic
.. _super_creator:
The filesystem creator is one of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0
- Linux
* - 1
- Hurd
* - 2
- Masix
* - 3
- FreeBSD
* - 4
- Lites
.. _super_revision:
The superblock revision is one of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0
- Original format
* - 1
- v2 format w/ dynamic inode sizes
Note that ``EXT4_DYNAMIC_REV`` refers to a revision 1 or newer filesystem.
.. _super_compat:
The superblock compatible features field is a combination of any of the
following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- Directory preallocation (COMPAT\_DIR\_PREALLOC).
* - 0x2
- “imagic inodes”. Not clear from the code what this does
(COMPAT\_IMAGIC\_INODES).
* - 0x4
- Has a journal (COMPAT\_HAS\_JOURNAL).
* - 0x8
- Supports extended attributes (COMPAT\_EXT\_ATTR).
* - 0x10
- Has reserved GDT blocks for filesystem expansion
(COMPAT\_RESIZE\_INODE). Requires RO\_COMPAT\_SPARSE\_SUPER.
* - 0x20
- Has directory indices (COMPAT\_DIR\_INDEX).
* - 0x40
- “Lazy BG”. Not in Linux kernel, seems to have been for uninitialized
block groups? (COMPAT\_LAZY\_BG)
* - 0x80
- “Exclude inode”. Not used. (COMPAT\_EXCLUDE\_INODE).
* - 0x100
- “Exclude bitmap”. Seems to be used to indicate the presence of
snapshot-related exclude bitmaps? Not defined in kernel or used in
e2fsprogs (COMPAT\_EXCLUDE\_BITMAP).
* - 0x200
- Sparse Super Block, v2. If this flag is set, the SB field s\_backup\_bgs
points to the two block groups that contain backup superblocks
(COMPAT\_SPARSE\_SUPER2).
.. _super_incompat:
The superblock incompatible features field is a combination of any of the
following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- Compression (INCOMPAT\_COMPRESSION).
* - 0x2
- Directory entries record the file type. See ext4\_dir\_entry\_2 below
(INCOMPAT\_FILETYPE).
* - 0x4
- Filesystem needs recovery (INCOMPAT\_RECOVER).
* - 0x8
- Filesystem has a separate journal device (INCOMPAT\_JOURNAL\_DEV).
* - 0x10
- Meta block groups. See the earlier discussion of this feature
(INCOMPAT\_META\_BG).
* - 0x40
- Files in this filesystem use extents (INCOMPAT\_EXTENTS).
* - 0x80
- Enable a filesystem size of 2^64 blocks (INCOMPAT\_64BIT).
* - 0x100
- Multiple mount protection. Not implemented (INCOMPAT\_MMP).
* - 0x200
- Flexible block groups. See the earlier discussion of this feature
(INCOMPAT\_FLEX\_BG).
* - 0x400
- Inodes can be used to store large extended attribute values
(INCOMPAT\_EA\_INODE).
* - 0x1000
- Data in directory entry (INCOMPAT\_DIRDATA). (Not implemented?)
* - 0x2000
- Metadata checksum seed is stored in the superblock. This feature enables
the administrator to change the UUID of a metadata\_csum filesystem
while the filesystem is mounted; without it, the checksum definition
requires all metadata blocks to be rewritten (INCOMPAT\_CSUM\_SEED).
* - 0x4000
- Large directory >2GB or 3-level htree (INCOMPAT\_LARGEDIR). Prior to
this feature, directories could not be larger than 4GiB and could not
have an htree more than 2 levels deep. If this feature is enabled,
directories can be larger than 4GiB and have a maximum htree depth of 3.
* - 0x8000
- Data in inode (INCOMPAT\_INLINE\_DATA).
* - 0x10000
- Encrypted inodes are present on the filesystem. (INCOMPAT\_ENCRYPT).
.. _super_rocompat:
The superblock read-only compatible features field is a combination of any of
the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x1
- Sparse superblocks. See the earlier discussion of this feature
(RO\_COMPAT\_SPARSE\_SUPER).
* - 0x2
- This filesystem has been used to store a file greater than 2GiB
(RO\_COMPAT\_LARGE\_FILE).
* - 0x4
- Not used in kernel or e2fsprogs (RO\_COMPAT\_BTREE\_DIR).
* - 0x8
- This filesystem has files whose sizes are represented in units of
logical blocks, not 512-byte sectors. This implies a very large file
indeed! (RO\_COMPAT\_HUGE\_FILE)
* - 0x10
- Group descriptors have checksums. In addition to detecting corruption,
this is useful for lazy formatting with uninitialized groups
(RO\_COMPAT\_GDT\_CSUM).
* - 0x20
- Indicates that the old ext3 32,000 subdirectory limit no longer applies
(RO\_COMPAT\_DIR\_NLINK). A directory's i\_links\_count will be set to 1
if it is incremented past 64,999.
* - 0x40
- Indicates that large inodes exist on this filesystem
(RO\_COMPAT\_EXTRA\_ISIZE).
* - 0x80
- This filesystem has a snapshot (RO\_COMPAT\_HAS\_SNAPSHOT).
* - 0x100
- `Quota <Quota>`__ (RO\_COMPAT\_QUOTA).
* - 0x200
- This filesystem supports “bigalloc”, which means that file extents are
tracked in units of clusters (of blocks) instead of blocks
(RO\_COMPAT\_BIGALLOC).
* - 0x400
- This filesystem supports metadata checksumming.
(RO\_COMPAT\_METADATA\_CSUM; implies RO\_COMPAT\_GDT\_CSUM, though
GDT\_CSUM must not be set)
* - 0x800
- Filesystem supports replicas. This feature is neither in the kernel nor
e2fsprogs. (RO\_COMPAT\_REPLICA)
* - 0x1000
- Read-only filesystem image; the kernel will not mount this image
read-write and most tools will refuse to write to the image.
(RO\_COMPAT\_READONLY)
* - 0x2000
- Filesystem tracks project quotas. (RO\_COMPAT\_PROJECT)
.. _super_def_hash:
The ``s_def_hash_version`` field is one of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0
- Legacy.
* - 0x1
- Half MD4.
* - 0x2
- Tea.
* - 0x3
- Legacy, unsigned.
* - 0x4
- Half MD4, unsigned.
* - 0x5
- Tea, unsigned.
.. _super_mountopts:
The ``s_default_mount_opts`` field is any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0001
- Print debugging info upon (re)mount. (EXT4\_DEFM\_DEBUG)
* - 0x0002
- New files take the gid of the containing directory (instead of the fsgid
of the current process). (EXT4\_DEFM\_BSDGROUPS)
* - 0x0004
- Support userspace-provided extended attributes. (EXT4\_DEFM\_XATTR\_USER)
* - 0x0008
- Support POSIX access control lists (ACLs). (EXT4\_DEFM\_ACL)
* - 0x0010
- Do not support 32-bit UIDs. (EXT4\_DEFM\_UID16)
* - 0x0020
- All data and metadata are commited to the journal.
(EXT4\_DEFM\_JMODE\_DATA)
* - 0x0040
- All data are flushed to the disk before metadata are committed to the
journal. (EXT4\_DEFM\_JMODE\_ORDERED)
* - 0x0060
- Data ordering is not preserved; data may be written after the metadata
has been written. (EXT4\_DEFM\_JMODE\_WBACK)
* - 0x0100
- Disable write flushes. (EXT4\_DEFM\_NOBARRIER)
* - 0x0200
- Track which blocks in a filesystem are metadata and therefore should not
be used as data blocks. This option will be enabled by default on 3.18,
hopefully. (EXT4\_DEFM\_BLOCK\_VALIDITY)
* - 0x0400
- Enable DISCARD support, where the storage device is told about blocks
becoming unused. (EXT4\_DEFM\_DISCARD)
* - 0x0800
- Disable delayed allocation. (EXT4\_DEFM\_NODELALLOC)
.. _super_flags:
The ``s_flags`` field is any combination of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0x0001
- Signed directory hash in use.
* - 0x0002
- Unsigned directory hash in use.
* - 0x0004
- To test development code.
.. _super_encrypt:
The ``s_encrypt_algos`` list can contain any of the following:
.. list-table::
:widths: 1 79
:header-rows: 1
* - Value
- Description
* - 0
- Invalid algorithm (ENCRYPTION\_MODE\_INVALID).
* - 1
- 256-bit AES in XTS mode (ENCRYPTION\_MODE\_AES\_256\_XTS).
* - 2
- 256-bit AES in GCM mode (ENCRYPTION\_MODE\_AES\_256\_GCM).
* - 3
- 256-bit AES in CBC mode (ENCRYPTION\_MODE\_AES\_256\_CBC).
Total size of the superblock is 1024 bytes.

11
Documentation/index.rst
View File

@ -102,6 +102,17 @@ implementation.
sh/index
Filesystem Documentation
------------------------
The documentation in this section are provided by specific filesystem
subprojects.
.. toctree::
:maxdepth: 2
filesystems/ext4/index
Korean translations
-------------------

10
fs/dax.c
View File

@ -566,7 +566,8 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
if (index >= end)
break;
if (!radix_tree_exceptional_entry(pvec_ent))
if (WARN_ON_ONCE(
!radix_tree_exceptional_entry(pvec_ent)))
continue;
xa_lock_irq(&mapping->i_pages);
@ -578,6 +579,13 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
if (page)
break;
}
/*
* We don't expect normal struct page entries to exist in our
* tree, but we keep these pagevec calls so that this code is
* consistent with the common pattern for handling pagevecs
* throughout the kernel.
*/
pagevec_remove_exceptionals(&pvec);
pagevec_release(&pvec);
index++;

6
fs/ext4/balloc.c
View File

@ -426,9 +426,9 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
}
bh = sb_getblk(sb, bitmap_blk);
if (unlikely(!bh)) {
ext4_error(sb, "Cannot get buffer for block bitmap - "
"block_group = %u, block_bitmap = %llu",
block_group, bitmap_blk);
ext4_warning(sb, "Cannot get buffer for block bitmap - "
"block_group = %u, block_bitmap = %llu",
block_group, bitmap_blk);
return ERR_PTR(-ENOMEM);
}

32
fs/ext4/ext4.h
View File

@ -789,17 +789,16 @@ struct move_extent {
* affected filesystem before 2242.
*/
static inline __le32 ext4_encode_extra_time(struct timespec *time)
static inline __le32 ext4_encode_extra_time(struct timespec64 *time)
{
u32 extra = sizeof(time->tv_sec) > 4 ?
((time->tv_sec - (s32)time->tv_sec) >> 32) & EXT4_EPOCH_MASK : 0;
u32 extra =((time->tv_sec - (s32)time->tv_sec) >> 32) & EXT4_EPOCH_MASK;
return cpu_to_le32(extra | (time->tv_nsec << EXT4_EPOCH_BITS));
}
static inline void ext4_decode_extra_time(struct timespec *time, __le32 extra)
static inline void ext4_decode_extra_time(struct timespec64 *time,
__le32 extra)
{
if (unlikely(sizeof(time->tv_sec) > 4 &&
(extra & cpu_to_le32(EXT4_EPOCH_MASK)))) {
if (unlikely(extra & cpu_to_le32(EXT4_EPOCH_MASK))) {
#if 1
/* Handle legacy encoding of pre-1970 dates with epoch
@ -821,9 +820,8 @@ static inline void ext4_decode_extra_time(struct timespec *time, __le32 extra)
do { \
(raw_inode)->xtime = cpu_to_le32((inode)->xtime.tv_sec); \
if (EXT4_FITS_IN_INODE(raw_inode, EXT4_I(inode), xtime ## _extra)) {\
struct timespec ts = timespec64_to_timespec((inode)->xtime); \
(raw_inode)->xtime ## _extra = \
ext4_encode_extra_time(&ts); \
ext4_encode_extra_time(&(inode)->xtime); \
} \
} while (0)
@ -840,10 +838,8 @@ do { \
do { \
(inode)->xtime.tv_sec = (signed)le32_to_cpu((raw_inode)->xtime); \
if (EXT4_FITS_IN_INODE(raw_inode, EXT4_I(inode), xtime ## _extra)) { \
struct timespec ts = timespec64_to_timespec((inode)->xtime); \
ext4_decode_extra_time(&ts, \
ext4_decode_extra_time(&(inode)->xtime, \
raw_inode->xtime ## _extra); \
(inode)->xtime = timespec_to_timespec64(ts); \
} \
else \
(inode)->xtime.tv_nsec = 0; \
@ -993,9 +989,9 @@ struct ext4_inode_info {
/*
* File creation time. Its function is same as that of
* struct timespec i_{a,c,m}time in the generic inode.
* struct timespec64 i_{a,c,m}time in the generic inode.
*/
struct timespec i_crtime;
struct timespec64 i_crtime;
/* mballoc */
struct list_head i_prealloc_list;
@ -1299,7 +1295,14 @@ struct ext4_super_block {
__le32 s_lpf_ino; /* Location of the lost+found inode */
__le32 s_prj_quota_inum; /* inode for tracking project quota */
__le32 s_checksum_seed; /* crc32c(uuid) if csum_seed set */
__le32 s_reserved[98]; /* Padding to the end of the block */
__u8 s_wtime_hi;
__u8 s_mtime_hi;
__u8 s_mkfs_time_hi;
__u8 s_lastcheck_hi;
__u8 s_first_error_time_hi;
__u8 s_last_error_time_hi;
__u8 s_pad[2];
__le32 s_reserved[96]; /* Padding to the end of the block */
__le32 s_checksum; /* crc32c(superblock) */
};
@ -2456,6 +2459,7 @@ extern int ext4_get_inode_loc(struct inode *, struct ext4_iloc *);
extern int ext4_inode_attach_jinode(struct inode *inode);
extern int ext4_can_truncate(struct inode *inode);
extern int ext4_truncate(struct inode *);
extern int ext4_break_layouts(struct inode *);
extern int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length);
extern int ext4_truncate_restart_trans(handle_t *, struct inode *, int nblocks);
extern void ext4_set_inode_flags(struct inode *);

17
fs/ext4/extents.c
View File

@ -4826,6 +4826,13 @@ static long ext4_zero_range(struct file *file, loff_t offset,
* released from page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
ret = ext4_break_layouts(inode);
if (ret) {
up_write(&EXT4_I(inode)->i_mmap_sem);
goto out_mutex;
}
ret = ext4_update_disksize_before_punch(inode, offset, len);
if (ret) {
up_write(&EXT4_I(inode)->i_mmap_sem);
@ -5499,6 +5506,11 @@ int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
ret = ext4_break_layouts(inode);
if (ret)
goto out_mmap;
/*
* Need to round down offset to be aligned with page size boundary
* for page size > block size.
@ -5647,6 +5659,11 @@ int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
ret = ext4_break_layouts(inode);
if (ret)
goto out_mmap;
/*
* Need to round down to align start offset to page size boundary
* for page size > block size.

8
fs/ext4/ialloc.c
View File

@ -138,9 +138,9 @@ ext4_read_inode_bitmap(struct super_block *sb, ext4_group_t block_group)
}
bh = sb_getblk(sb, bitmap_blk);
if (unlikely(!bh)) {
ext4_error(sb, "Cannot read inode bitmap - "
"block_group = %u, inode_bitmap = %llu",
block_group, bitmap_blk);
ext4_warning(sb, "Cannot read inode bitmap - "
"block_group = %u, inode_bitmap = %llu",
block_group, bitmap_blk);
return ERR_PTR(-ENOMEM);
}
if (bitmap_uptodate(bh))
@ -1086,7 +1086,7 @@ got:
/* This is the optimal IO size (for stat), not the fs block size */
inode->i_blocks = 0;
inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
ei->i_crtime = timespec64_to_timespec(inode->i_mtime);
ei->i_crtime = inode->i_mtime;
memset(ei->i_data, 0, sizeof(ei->i_data));
ei->i_dir_start_lookup = 0;

65
fs/ext4/inode.c
View File

@ -317,7 +317,7 @@ stop_handle:
* (Well, we could do this if we need to, but heck - it works)
*/
ext4_orphan_del(handle, inode);
EXT4_I(inode)->i_dtime = get_seconds();
EXT4_I(inode)->i_dtime = (__u32)ktime_get_real_seconds();
/*
* One subtle ordering requirement: if anything has gone wrong
@ -4191,6 +4191,39 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset,
return 0;
}
static void ext4_wait_dax_page(struct ext4_inode_info *ei, bool *did_unlock)
{
*did_unlock = true;
up_write(&ei->i_mmap_sem);
schedule();
down_write(&ei->i_mmap_sem);
}
int ext4_break_layouts(struct inode *inode)
{
struct ext4_inode_info *ei = EXT4_I(inode);
struct page *page;
bool retry;
int error;
if (WARN_ON_ONCE(!rwsem_is_locked(&ei->i_mmap_sem)))
return -EINVAL;
do {
retry = false;
page = dax_layout_busy_page(inode->i_mapping);
if (!page)
return 0;
error = ___wait_var_event(&page->_refcount,
atomic_read(&page->_refcount) == 1,
TASK_INTERRUPTIBLE, 0, 0,
ext4_wait_dax_page(ei, &retry));
} while (error == 0 && retry);
return error;
}
/*
* ext4_punch_hole: punches a hole in a file by releasing the blocks
* associated with the given offset and length
@ -4264,6 +4297,11 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
* page cache.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
ret = ext4_break_layouts(inode);
if (ret)
goto out_dio;
first_block_offset = round_up(offset, sb->s_blocksize);
last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;
@ -4944,17 +4982,14 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
ret = -EFSCORRUPTED;
goto bad_inode;
} else if (!ext4_has_inline_data(inode)) {
if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
if ((S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode) ||
(S_ISLNK(inode->i_mode) &&
!ext4_inode_is_fast_symlink(inode))))
/* Validate extent which is part of inode */
/* validate the block references in the inode */
if (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode) ||
(S_ISLNK(inode->i_mode) &&
!ext4_inode_is_fast_symlink(inode))) {
if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
ret = ext4_ext_check_inode(inode);
} else if (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode) ||
(S_ISLNK(inode->i_mode) &&
!ext4_inode_is_fast_symlink(inode))) {
/* Validate block references which are part of inode */
ret = ext4_ind_check_inode(inode);
else
ret = ext4_ind_check_inode(inode);
}
}
if (ret)
@ -5553,6 +5588,14 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
ext4_wait_for_tail_page_commit(inode);
}
down_write(&EXT4_I(inode)->i_mmap_sem);
rc = ext4_break_layouts(inode);
if (rc) {
up_write(&EXT4_I(inode)->i_mmap_sem);
error = rc;
goto err_out;
}
/*
* Truncate pagecache after we've waited for commit
* in data=journal mode to make pages freeable.

7
fs/ext4/mballoc.c
View File

@ -14,6 +14,7 @@
#include <linux/log2.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/nospec.h>
#include <linux/backing-dev.h>
#include <trace/events/ext4.h>
@ -2140,7 +2141,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
* This should tell if fe_len is exactly power of 2
*/
if ((ac->ac_g_ex.fe_len & (~(1 << (i - 1)))) == 0)
ac->ac_2order = i - 1;
ac->ac_2order = array_index_nospec(i - 1,
sb->s_blocksize_bits + 2);
}
/* if stream allocation is enabled, use global goal */
@ -3799,7 +3801,6 @@ ext4_mb_release_inode_pa(struct ext4_buddy *e4b, struct buffer_head *bitmap_bh,
ext4_group_t group;
ext4_grpblk_t bit;
unsigned long long grp_blk_start;
int err = 0;
int free = 0;
BUG_ON(pa->pa_deleted == 0);
@ -3840,7 +3841,7 @@ ext4_mb_release_inode_pa(struct ext4_buddy *e4b, struct buffer_head *bitmap_bh,
}
atomic_add(free, &sbi->s_mb_discarded);
return err;
return 0;
}
static noinline_for_stack int

6
fs/ext4/mmp.c
View File

@ -147,7 +147,7 @@ static int kmmpd(void *data)
mmp_block = le64_to_cpu(es->s_mmp_block);
mmp = (struct mmp_struct *)(bh->b_data);
mmp->mmp_time = cpu_to_le64(get_seconds());
mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
/*
* Start with the higher mmp_check_interval and reduce it if
* the MMP block is being updated on time.
@ -165,7 +165,7 @@ static int kmmpd(void *data)
seq = 1;
mmp->mmp_seq = cpu_to_le32(seq);
mmp->mmp_time = cpu_to_le64(get_seconds());
mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
last_update_time = jiffies;
retval = write_mmp_block(sb, bh);
@ -241,7 +241,7 @@ static int kmmpd(void *data)
* Unmount seems to be clean.
*/
mmp->mmp_seq = cpu_to_le32(EXT4_MMP_SEQ_CLEAN);
mmp->mmp_time = cpu_to_le64(get_seconds());
mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
retval = write_mmp_block(sb, bh);

4
fs/ext4/move_extent.c
View File

@ -134,9 +134,7 @@ mext_page_double_lock(struct inode *inode1, struct inode *inode2,
mapping[0] = inode1->i_mapping;
mapping[1] = inode2->i_mapping;
} else {
pgoff_t tmp = index1;
index1 = index2;
index2 = tmp;
swap(index1, index2);
mapping[0] = inode2->i_mapping;
mapping[1] = inode1->i_mapping;
}

1
fs/ext4/namei.c
View File

@ -1398,6 +1398,7 @@ static struct buffer_head * ext4_find_entry (struct inode *dir,
goto cleanup_and_exit;
dxtrace(printk(KERN_DEBUG "ext4_find_entry: dx failed, "
"falling back\n"));
ret = NULL;
}
nblocks = dir->i_size >> EXT4_BLOCK_SIZE_BITS(sb);
if (!nblocks) {

70
fs/ext4/super.c
View File

@ -312,6 +312,24 @@ void ext4_itable_unused_set(struct super_block *sb,
bg->bg_itable_unused_hi = cpu_to_le16(count >> 16);
}
static void __ext4_update_tstamp(__le32 *lo, __u8 *hi)
{
time64_t now = ktime_get_real_seconds();
now = clamp_val(now, 0, (1ull << 40) - 1);
*lo = cpu_to_le32(lower_32_bits(now));
*hi = upper_32_bits(now);
}
static time64_t __ext4_get_tstamp(__le32 *lo, __u8 *hi)
{
return ((time64_t)(*hi) << 32) + le32_to_cpu(*lo);
}
#define ext4_update_tstamp(es, tstamp) \
__ext4_update_tstamp(&(es)->tstamp, &(es)->tstamp ## _hi)
#define ext4_get_tstamp(es, tstamp) \
__ext4_get_tstamp(&(es)->tstamp, &(es)->tstamp ## _hi)
static void __save_error_info(struct super_block *sb, const char *func,
unsigned int line)
@ -322,11 +340,12 @@ static void __save_error_info(struct super_block *sb, const char *func,
if (bdev_read_only(sb->s_bdev))
return;
es->s_state |= cpu_to_le16(EXT4_ERROR_FS);
es->s_last_error_time = cpu_to_le32(get_seconds());
ext4_update_tstamp(es, s_last_error_time);
strncpy(es->s_last_error_func, func, sizeof(es->s_last_error_func));
es->s_last_error_line = cpu_to_le32(line);
if (!es->s_first_error_time) {
es->s_first_error_time = es->s_last_error_time;
es->s_first_error_time_hi = es->s_last_error_time_hi;
strncpy(es->s_first_error_func, func,
sizeof(es->s_first_error_func));
es->s_first_error_line = cpu_to_le32(line);
@ -776,26 +795,26 @@ void ext4_mark_group_bitmap_corrupted(struct super_block *sb,
struct ext4_sb_info *sbi = EXT4_SB(sb);
struct ext4_group_info *grp = ext4_get_group_info(sb, group);
struct ext4_group_desc *gdp = ext4_get_group_desc(sb, group, NULL);
int ret;
if ((flags & EXT4_GROUP_INFO_BBITMAP_CORRUPT) &&
!EXT4_MB_GRP_BBITMAP_CORRUPT(grp)) {
percpu_counter_sub(&sbi->s_freeclusters_counter,
grp->bb_free);
set_bit(EXT4_GROUP_INFO_BBITMAP_CORRUPT_BIT,
&grp->bb_state);
if (flags & EXT4_GROUP_INFO_BBITMAP_CORRUPT) {
ret = ext4_test_and_set_bit(EXT4_GROUP_INFO_BBITMAP_CORRUPT_BIT,
&grp->bb_state);
if (!ret)
percpu_counter_sub(&sbi->s_freeclusters_counter,
grp->bb_free);
}
if ((flags & EXT4_GROUP_INFO_IBITMAP_CORRUPT) &&
!EXT4_MB_GRP_IBITMAP_CORRUPT(grp)) {
if (gdp) {
if (flags & EXT4_GROUP_INFO_IBITMAP_CORRUPT) {
ret = ext4_test_and_set_bit(EXT4_GROUP_INFO_IBITMAP_CORRUPT_BIT,
&grp->bb_state);
if (!ret && gdp) {
int count;
count = ext4_free_inodes_count(sb, gdp);
percpu_counter_sub(&sbi->s_freeinodes_counter,
count);
}
set_bit(EXT4_GROUP_INFO_IBITMAP_CORRUPT_BIT,
&grp->bb_state);
}
}
@ -2174,8 +2193,8 @@ static int ext4_setup_super(struct super_block *sb, struct ext4_super_block *es,
"warning: maximal mount count reached, "
"running e2fsck is recommended");
else if (le32_to_cpu(es->s_checkinterval) &&
(le32_to_cpu(es->s_lastcheck) +
le32_to_cpu(es->s_checkinterval) <= get_seconds()))
(ext4_get_tstamp(es, s_lastcheck) +
le32_to_cpu(es->s_checkinterval) <= ktime_get_real_seconds()))
ext4_msg(sb, KERN_WARNING,
"warning: checktime reached, "
"running e2fsck is recommended");
@ -2184,7 +2203,7 @@ static int ext4_setup_super(struct super_block *sb, struct ext4_super_block *es,
if (!(__s16) le16_to_cpu(es->s_max_mnt_count))
es->s_max_mnt_count = cpu_to_le16(EXT4_DFL_MAX_MNT_COUNT);
le16_add_cpu(&es->s_mnt_count, 1);
es->s_mtime = cpu_to_le32(get_seconds());
ext4_update_tstamp(es, s_mtime);
ext4_update_dynamic_rev(sb);
if (sbi->s_journal)
ext4_set_feature_journal_needs_recovery(sb);
@ -2875,8 +2894,9 @@ static void print_daily_error_info(struct timer_list *t)
ext4_msg(sb, KERN_NOTICE, "error count since last fsck: %u",
le32_to_cpu(es->s_error_count));
if (es->s_first_error_time) {
printk(KERN_NOTICE "EXT4-fs (%s): initial error at time %u: %.*s:%d",
sb->s_id, le32_to_cpu(es->s_first_error_time),
printk(KERN_NOTICE "EXT4-fs (%s): initial error at time %llu: %.*s:%d",
sb->s_id,
ext4_get_tstamp(es, s_first_error_time),
(int) sizeof(es->s_first_error_func),
es->s_first_error_func,
le32_to_cpu(es->s_first_error_line));
@ -2889,8 +2909,9 @@ static void print_daily_error_info(struct timer_list *t)
printk(KERN_CONT "\n");
}
if (es->s_last_error_time) {
printk(KERN_NOTICE "EXT4-fs (%s): last error at time %u: %.*s:%d",
sb->s_id, le32_to_cpu(es->s_last_error_time),
printk(KERN_NOTICE "EXT4-fs (%s): last error at time %llu: %.*s:%d",
sb->s_id,
ext4_get_tstamp(es, s_last_error_time),
(int) sizeof(es->s_last_error_func),
es->s_last_error_func,
le32_to_cpu(es->s_last_error_line));
@ -4813,7 +4834,7 @@ static int ext4_commit_super(struct super_block *sb, int sync)
* to complain and force a full file system check.
*/
if (!(sb->s_flags & SB_RDONLY))
es->s_wtime = cpu_to_le32(get_seconds());
ext4_update_tstamp(es, s_wtime);
if (sb->s_bdev->bd_part)
es->s_kbytes_written =
cpu_to_le64(EXT4_SB(sb)->s_kbytes_written +
@ -5080,6 +5101,9 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
#endif
char *orig_data = kstrdup(data, GFP_KERNEL);
if (data && !orig_data)
return -ENOMEM;
/* Store the original options */
old_sb_flags = sb->s_flags;
old_opts.s_mount_opt = sbi->s_mount_opt;
@ -5665,13 +5689,13 @@ static int ext4_enable_quotas(struct super_block *sb)
DQUOT_USAGE_ENABLED |
(quota_mopt[type] ? DQUOT_LIMITS_ENABLED : 0));
if (err) {
for (type--; type >= 0; type--)
dquot_quota_off(sb, type);
ext4_warning(sb,
"Failed to enable quota tracking "
"(type=%d, err=%d). Please run "
"e2fsck to fix.", type, err);
for (type--; type >= 0; type--)
dquot_quota_off(sb, type);
return err;
}
}

32
fs/ext4/sysfs.c
View File

@ -25,6 +25,8 @@ typedef enum {
attr_reserved_clusters,
attr_inode_readahead,
attr_trigger_test_error,
attr_first_error_time,
attr_last_error_time,
attr_feature,
attr_pointer_ui,
attr_pointer_atomic,
@ -182,8 +184,8 @@ EXT4_RW_ATTR_SBI_UI(warning_ratelimit_burst, s_warning_ratelimit_state.burst);
EXT4_RW_ATTR_SBI_UI(msg_ratelimit_interval_ms, s_msg_ratelimit_state.interval);
EXT4_RW_ATTR_SBI_UI(msg_ratelimit_burst, s_msg_ratelimit_state.burst);
EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
EXT4_RO_ATTR_ES_UI(first_error_time, s_first_error_time);
EXT4_RO_ATTR_ES_UI(last_error_time, s_last_error_time);
EXT4_ATTR(first_error_time, 0444, first_error_time);
EXT4_ATTR(last_error_time, 0444, last_error_time);
static unsigned int old_bump_val = 128;
EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
@ -249,6 +251,15 @@ static void *calc_ptr(struct ext4_attr *a, struct ext4_sb_info *sbi)
return NULL;
}
static ssize_t __print_tstamp(char *buf, __le32 lo, __u8 hi)
{
return snprintf(buf, PAGE_SIZE, "%lld",
((time64_t)hi << 32) + le32_to_cpu(lo));
}
#define print_tstamp(buf, es, tstamp) \
__print_tstamp(buf, (es)->tstamp, (es)->tstamp ## _hi)
static ssize_t ext4_attr_show(struct kobject *kobj,
struct attribute *attr, char *buf)
{
@ -274,8 +285,12 @@ static ssize_t ext4_attr_show(struct kobject *kobj,
case attr_pointer_ui:
if (!ptr)
return 0;
return snprintf(buf, PAGE_SIZE, "%u\n",
*((unsigned int *) ptr));
if (a->attr_ptr == ptr_ext4_super_block_offset)
return snprintf(buf, PAGE_SIZE, "%u\n",
le32_to_cpup(ptr));
else
return snprintf(buf, PAGE_SIZE, "%u\n",
*((unsigned int *) ptr));
case attr_pointer_atomic:
if (!ptr)
return 0;
@ -283,6 +298,10 @@ static ssize_t ext4_attr_show(struct kobject *kobj,
atomic_read((atomic_t *) ptr));
case attr_feature:
return snprintf(buf, PAGE_SIZE, "supported\n");
case attr_first_error_time:
return print_tstamp(buf, sbi->s_es, s_first_error_time);
case attr_last_error_time:
return print_tstamp(buf, sbi->s_es, s_last_error_time);
}
return 0;
@ -308,7 +327,10 @@ static ssize_t ext4_attr_store(struct kobject *kobj,
ret = kstrtoul(skip_spaces(buf), 0, &t);
if (ret)
return ret;
*((unsigned int *) ptr) = t;
if (a->attr_ptr == ptr_ext4_super_block_offset)
*((__le32 *) ptr) = cpu_to_le32(t);
else
*((unsigned int *) ptr) = t;
return len;
case attr_inode_readahead:
return inode_readahead_blks_store(sbi, buf, len);

4
fs/ext4/truncate.h
View File

@ -11,6 +11,10 @@
*/
static inline void ext4_truncate_failed_write(struct inode *inode)
{
/*
* We don't need to call ext4_break_layouts() because the blocks we
* are truncating were never visible to userspace.
*/
down_write(&EXT4_I(inode)->i_mmap_sem);
truncate_inode_pages(inode->i_mapping, inode->i_size);
ext4_truncate(inode);

2
fs/ext4/xattr.c
View File

@ -190,6 +190,8 @@ ext4_xattr_check_entries(struct ext4_xattr_entry *entry, void *end,
struct ext4_xattr_entry *next = EXT4_XATTR_NEXT(e);
if ((void *)next >= end)
return -EFSCORRUPTED;
if (strnlen(e->e_name, e->e_name_len) != e->e_name_len)
return -EFSCORRUPTED;
e = next;
}

3
fs/jbd2/commit.c
View File

@ -121,7 +121,7 @@ static int journal_submit_commit_record(journal_t *journal,
struct commit_header *tmp;
struct buffer_head *bh;
int ret;
struct timespec64 now = current_kernel_time64();
struct timespec64 now;
*cbh = NULL;
@ -134,6 +134,7 @@ static int journal_submit_commit_record(journal_t *journal,
return 1;
tmp = (struct commit_header *)bh->b_data;
ktime_get_coarse_real_ts64(&now);
tmp->h_commit_sec = cpu_to_be64(now.tv_sec);
tmp->h_commit_nsec = cpu_to_be32(now.tv_nsec);