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linux/Documentation/admin-guide/pstore-blk.rst
Christoph Hellwig b6f8ed33ab pstore/blk: remove {un,}register_pstore_blk 
This interface is entirely unused, so remove them and various bits of
unreachable code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201016132047.3068029-4-hch@lst.de
2020-12-01 12:01:03 -08:00

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.. SPDX-License-Identifier: GPL-2.0
pstore block oops/panic logger
==============================
Introduction
------------
pstore block (pstore/blk) is an oops/panic logger that writes its logs to a
block device and non-block device before the system crashes. You can get
these log files by mounting pstore filesystem like::
mount -t pstore pstore /sys/fs/pstore
pstore block concepts
---------------------
pstore/blk provides efficient configuration method for pstore/blk, which
divides all configurations into two parts, configurations for user and
configurations for driver.
Configurations for user determine how pstore/blk works, such as pmsg_size,
kmsg_size and so on. All of them support both Kconfig and module parameters,
but module parameters have priority over Kconfig.
Configurations for driver are all about block device and non-block device,
such as total_size of block device and read/write operations.
Configurations for user
-----------------------
All of these configurations support both Kconfig and module parameters, but
module parameters have priority over Kconfig.
Here is an example for module parameters::
pstore_blk.blkdev=/dev/mmcblk0p7 pstore_blk.kmsg_size=64 best_effort=y
The detail of each configurations may be of interest to you.
blkdev
~~~~~~
The block device to use. Most of the time, it is a partition of block device.
It's required for pstore/blk. It is also used for MTD device.
It accepts the following variants for block device:
1. <hex_major><hex_minor> device number in hexadecimal represents itself; no
leading 0x, for example b302.
#. /dev/<disk_name> represents the device number of disk
#. /dev/<disk_name><decimal> represents the device number of partition - device
number of disk plus the partition number
#. /dev/<disk_name>p<decimal> - same as the above; this form is used when disk
name of partitioned disk ends with a digit.
#. PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF represents the unique id of
a partition if the partition table provides it. The UUID may be either an
EFI/GPT UUID, or refer to an MSDOS partition using the format SSSSSSSS-PP,
where SSSSSSSS is a zero-filled hex representation of the 32-bit
"NT disk signature", and PP is a zero-filled hex representation of the
1-based partition number.
#. PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to a
partition with a known unique id.
#. <major>:<minor> major and minor number of the device separated by a colon.
It accepts the following variants for MTD device:
1. <device name> MTD device name. "pstore" is recommended.
#. <device number> MTD device number.
kmsg_size
~~~~~~~~~
The chunk size in KB for oops/panic front-end. It **MUST** be a multiple of 4.
It's optional if you do not care oops/panic log.
There are multiple chunks for oops/panic front-end depending on the remaining
space except other pstore front-ends.
pstore/blk will log to oops/panic chunks one by one, and always overwrite the
oldest chunk if there is no more free chunk.
pmsg_size
~~~~~~~~~
The chunk size in KB for pmsg front-end. It **MUST** be a multiple of 4.
It's optional if you do not care pmsg log.
Unlike oops/panic front-end, there is only one chunk for pmsg front-end.
Pmsg is a user space accessible pstore object. Writes to */dev/pmsg0* are
appended to the chunk. On reboot the contents are available in
*/sys/fs/pstore/pmsg-pstore-blk-0*.
console_size
~~~~~~~~~~~~
The chunk size in KB for console front-end. It **MUST** be a multiple of 4.
It's optional if you do not care console log.
Similar to pmsg front-end, there is only one chunk for console front-end.
All log of console will be appended to the chunk. On reboot the contents are
available in */sys/fs/pstore/console-pstore-blk-0*.
ftrace_size
~~~~~~~~~~~
The chunk size in KB for ftrace front-end. It **MUST** be a multiple of 4.
It's optional if you do not care console log.
Similar to oops front-end, there are multiple chunks for ftrace front-end
depending on the count of cpu processors. Each chunk size is equal to
ftrace_size / processors_count.
All log of ftrace will be appended to the chunk. On reboot the contents are
combined and available in */sys/fs/pstore/ftrace-pstore-blk-0*.
Persistent function tracing might be useful for debugging software or hardware
related hangs. Here is an example of usage::
# mount -t pstore pstore /sys/fs/pstore
# mount -t debugfs debugfs /sys/kernel/debug/
# echo 1 > /sys/kernel/debug/pstore/record_ftrace
# reboot -f
[...]
# mount -t pstore pstore /sys/fs/pstore
# tail /sys/fs/pstore/ftrace-pstore-blk-0
CPU:0 ts:5914676 c0063828 c0063b94 call_cpuidle <- cpu_startup_entry+0x1b8/0x1e0
CPU:0 ts:5914678 c039ecdc c006385c cpuidle_enter_state <- call_cpuidle+0x44/0x48
CPU:0 ts:5914680 c039e9a0 c039ecf0 cpuidle_enter_freeze <- cpuidle_enter_state+0x304/0x314
CPU:0 ts:5914681 c0063870 c039ea30 sched_idle_set_state <- cpuidle_enter_state+0x44/0x314
CPU:1 ts:5916720 c0160f59 c015ee04 kernfs_unmap_bin_file <- __kernfs_remove+0x140/0x204
CPU:1 ts:5916721 c05ca625 c015ee0c __mutex_lock_slowpath <- __kernfs_remove+0x148/0x204
CPU:1 ts:5916723 c05c813d c05ca630 yield_to <- __mutex_lock_slowpath+0x314/0x358
CPU:1 ts:5916724 c05ca2d1 c05ca638 __ww_mutex_lock <- __mutex_lock_slowpath+0x31c/0x358
max_reason
~~~~~~~~~~
Limiting which kinds of kmsg dumps are stored can be controlled via
the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
otherwise KMSG_DUMP_MAX.
Configurations for driver
-------------------------
A device driver uses ``register_pstore_device`` with
``struct pstore_device_info`` to register to pstore/blk.
.. kernel-doc:: fs/pstore/blk.c
:export:
Compression and header
----------------------
Block device is large enough for uncompressed oops data. Actually we do not
recommend data compression because pstore/blk will insert some information into
the first line of oops/panic data. For example::
Panic: Total 16 times
It means that it's OOPS|Panic for the 16th time since the first booting.
Sometimes the number of occurrences of oops|panic since the first booting is
important to judge whether the system is stable.
The following line is inserted by pstore filesystem. For example::
Oops#2 Part1
It means that it's OOPS for the 2nd time on the last boot.
Reading the data
----------------
The dump data can be read from the pstore filesystem. The format for these
files is ``dmesg-pstore-blk-[N]`` for oops/panic front-end,
``pmsg-pstore-blk-0`` for pmsg front-end and so on. The timestamp of the
dump file records the trigger time. To delete a stored record from block
device, simply unlink the respective pstore file.
Attentions in panic read/write APIs
-----------------------------------
If on panic, the kernel is not going to run for much longer, the tasks will not
be scheduled and most kernel resources will be out of service. It
looks like a single-threaded program running on a single-core computer.
The following points require special attention for panic read/write APIs:
1. Can **NOT** allocate any memory.
If you need memory, just allocate while the block driver is initializing
rather than waiting until the panic.
#. Must be polled, **NOT** interrupt driven.
No task schedule any more. The block driver should delay to ensure the write
succeeds, but NOT sleep.
#. Can **NOT** take any lock.
There is no other task, nor any shared resource; you are safe to break all
locks.
#. Just use CPU to transfer.
Do not use DMA to transfer unless you are sure that DMA will not keep lock.
#. Control registers directly.
Please control registers directly rather than use Linux kernel resources.
Do I/O map while initializing rather than wait until a panic occurs.
#. Reset your block device and controller if necessary.
If you are not sure of the state of your block device and controller when
a panic occurs, you are safe to stop and reset them.
pstore/blk supports psblk_blkdev_info(), which is defined in
*linux/pstore_blk.h*, to get information of using block device, such as the
device number, sector count and start sector of the whole disk.
pstore block internals
----------------------
For developer reference, here are all the important structures and APIs:
.. kernel-doc:: fs/pstore/zone.c
:internal:
.. kernel-doc:: include/linux/pstore_zone.h
:internal:
.. kernel-doc:: fs/pstore/blk.c
:internal:
.. kernel-doc:: include/linux/pstore_blk.h
:internal:
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