c334730988
When doing a buffered write, through one of the write family syscalls, we look for ranges which currently don't have allocated extents and set the 'delalloc new' bit on them, so that we can report a correct number of used blocks to the stat(2) syscall until delalloc is flushed and ordered extents complete. However there are a few other places where we can do a buffered write against a range that is mapped to a hole (no extent allocated) and where we do not set the 'new delalloc' bit. Those places are: - Doing a memory mapped write against a hole; - Cloning an inline extent into a hole starting at file offset 0; - Calling btrfs_cont_expand() when the i_size of the file is not aligned to the sector size and is located in a hole. For example when cloning to a destination offset beyond EOF. So after such cases, until the corresponding delalloc range is flushed and the respective ordered extents complete, we can report an incorrect number of blocks used through the stat(2) syscall. In some cases we can end up reporting 0 used blocks to stat(2), which is a particular bad value to report as it may mislead tools to think a file is completely sparse when its i_size is not zero, making them skip reading any data, an undesired consequence for tools such as archivers and other backup tools, as reported a long time ago in the following thread (and other past threads): https://lists.gnu.org/archive/html/bug-tar/2016-07/msg00001.html Example reproducer: $ cat reproducer.sh #!/bin/bash MNT=/mnt/sdi DEV=/dev/sdi mkfs.btrfs -f $DEV > /dev/null # mkfs.xfs -f $DEV > /dev/null # mkfs.ext4 -F $DEV > /dev/null # mkfs.f2fs -f $DEV > /dev/null mount $DEV $MNT xfs_io -f -c "truncate 64K" \ -c "mmap -w 0 64K" \ -c "mwrite -S 0xab 0 64K" \ -c "munmap" \ $MNT/foo blocks_used=$(stat -c %b $MNT/foo) echo "blocks used: $blocks_used" if [ $blocks_used -eq 0 ]; then echo "ERROR: blocks used is 0" fi umount $DEV $ ./reproducer.sh blocks used: 0 ERROR: blocks used is 0 So move the logic that decides to set the 'delalloc bit' bit into the function btrfs_set_extent_delalloc(), since that is what we use for all those missing cases as well as for the cases that currently work well. This change is also preparatory work for an upcoming patch that fixes other problems related to tracking and reporting the number of bytes used by an inode. CC: stable@vger.kernel.org # 4.19+ Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.