Documentation/admin-guide/pstore-blk.rst - linux/kernel/git/viro/vfs - Git at Google

 .. 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.

 When pstore/blk is built as a module, "blkdev" accepts the following variants:

 1. /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.

 When pstore/blk is built into the kernel, "blkdev" accepts the following variants:

 #. <hex_major><hex_minor> device number in hexadecimal representation,
    with no leading 0x, for example b302.
 #. 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 about the 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 about the 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 about the 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 about the ftrace 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:: include/linux/pstore_blk.h
    :internal:
	.. 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.

	When pstore/blk is built as a module, "blkdev" accepts the following variants:

	1. /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.

	When pstore/blk is built into the kernel, "blkdev" accepts the following variants:

	#. <hex_major><hex_minor> device number in hexadecimal representation,
	with no leading 0x, for example b302.
	#. 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 about the 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 about the 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 about the 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 about the ftrace 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:: include/linux/pstore_blk.h
	:internal: