.. SPDX-License-Identifier: GPL-2.0
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pstore block oops/panic logger
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==============================
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Introduction
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------------
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pstore block (pstore/blk) is an oops/panic logger that writes its logs to a
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block device and non-block device before the system crashes. You can get
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these log files by mounting pstore filesystem like::
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mount -t pstore pstore /sys/fs/pstore
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pstore block concepts
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---------------------
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pstore/blk provides efficient configuration method for pstore/blk, which
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divides all configurations into two parts, configurations for user and
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configurations for driver.
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Configurations for user determine how pstore/blk works, such as pmsg_size,
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kmsg_size and so on. All of them support both Kconfig and module parameters,
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but module parameters have priority over Kconfig.
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Configurations for driver are all about block device and non-block device,
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such as total_size of block device and read/write operations.
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Configurations for user
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-----------------------
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All of these configurations support both Kconfig and module parameters, but
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module parameters have priority over Kconfig.
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Here is an example for module parameters::
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pstore_blk.blkdev=179:7 pstore_blk.kmsg_size=64
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The detail of each configurations may be of interest to you.
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blkdev
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~~~~~~
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The block device to use. Most of the time, it is a partition of block device.
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It's required for pstore/blk. It is also used for MTD device.
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It accepts the following variants for block device:
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1. <hex_major><hex_minor> device number in hexadecimal represents itself; no
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leading 0x, for example b302.
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#. /dev/<disk_name> represents the device number of disk
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#. /dev/<disk_name><decimal> represents the device number of partition - device
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number of disk plus the partition number
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#. /dev/<disk_name>p<decimal> - same as the above; this form is used when disk
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name of partitioned disk ends with a digit.
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#. PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF represents the unique id of
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a partition if the partition table provides it. The UUID may be either an
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EFI/GPT UUID, or refer to an MSDOS partition using the format SSSSSSSS-PP,
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where SSSSSSSS is a zero-filled hex representation of the 32-bit
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"NT disk signature", and PP is a zero-filled hex representation of the
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1-based partition number.
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#. PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to a
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partition with a known unique id.
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#. <major>:<minor> major and minor number of the device separated by a colon.
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It accepts the following variants for MTD device:
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1. <device name> MTD device name. "pstore" is recommended.
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#. <device number> MTD device number.
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kmsg_size
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~~~~~~~~~
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The chunk size in KB for oops/panic front-end. It **MUST** be a multiple of 4.
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It's optional if you do not care oops/panic log.
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There are multiple chunks for oops/panic front-end depending on the remaining
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space except other pstore front-ends.
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pstore/blk will log to oops/panic chunks one by one, and always overwrite the
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oldest chunk if there is no more free chunk.
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pmsg_size
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~~~~~~~~~
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The chunk size in KB for pmsg front-end. It **MUST** be a multiple of 4.
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It's optional if you do not care pmsg log.
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Unlike oops/panic front-end, there is only one chunk for pmsg front-end.
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Pmsg is a user space accessible pstore object. Writes to */dev/pmsg0* are
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appended to the chunk. On reboot the contents are available in
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*/sys/fs/pstore/pmsg-pstore-blk-0*.
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console_size
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~~~~~~~~~~~~
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The chunk size in KB for console front-end. It **MUST** be a multiple of 4.
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It's optional if you do not care console log.
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Similar to pmsg front-end, there is only one chunk for console front-end.
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All log of console will be appended to the chunk. On reboot the contents are
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available in */sys/fs/pstore/console-pstore-blk-0*.
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ftrace_size
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~~~~~~~~~~~
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The chunk size in KB for ftrace front-end. It **MUST** be a multiple of 4.
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It's optional if you do not care console log.
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Similar to oops front-end, there are multiple chunks for ftrace front-end
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depending on the count of cpu processors. Each chunk size is equal to
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ftrace_size / processors_count.
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All log of ftrace will be appended to the chunk. On reboot the contents are
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combined and available in */sys/fs/pstore/ftrace-pstore-blk-0*.
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Persistent function tracing might be useful for debugging software or hardware
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related hangs. Here is an example of usage::
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# mount -t pstore pstore /sys/fs/pstore
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# mount -t debugfs debugfs /sys/kernel/debug/
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# echo 1 > /sys/kernel/debug/pstore/record_ftrace
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# reboot -f
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[...]
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# mount -t pstore pstore /sys/fs/pstore
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# tail /sys/fs/pstore/ftrace-pstore-blk-0
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CPU:0 ts:5914676 c0063828 c0063b94 call_cpuidle <- cpu_startup_entry+0x1b8/0x1e0
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CPU:0 ts:5914678 c039ecdc c006385c cpuidle_enter_state <- call_cpuidle+0x44/0x48
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CPU:0 ts:5914680 c039e9a0 c039ecf0 cpuidle_enter_freeze <- cpuidle_enter_state+0x304/0x314
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CPU:0 ts:5914681 c0063870 c039ea30 sched_idle_set_state <- cpuidle_enter_state+0x44/0x314
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CPU:1 ts:5916720 c0160f59 c015ee04 kernfs_unmap_bin_file <- __kernfs_remove+0x140/0x204
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CPU:1 ts:5916721 c05ca625 c015ee0c __mutex_lock_slowpath <- __kernfs_remove+0x148/0x204
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CPU:1 ts:5916723 c05c813d c05ca630 yield_to <- __mutex_lock_slowpath+0x314/0x358
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CPU:1 ts:5916724 c05ca2d1 c05ca638 __ww_mutex_lock <- __mutex_lock_slowpath+0x31c/0x358
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max_reason
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~~~~~~~~~~
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Limiting which kinds of kmsg dumps are stored can be controlled via
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the ``max_reason`` value, as defined in include/linux/kmsg_dump.h's
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``enum kmsg_dump_reason``. For example, to store both Oopses and Panics,
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``max_reason`` should be set to 2 (KMSG_DUMP_OOPS), to store only Panics
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``max_reason`` should be set to 1 (KMSG_DUMP_PANIC). Setting this to 0
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(KMSG_DUMP_UNDEF), means the reason filtering will be controlled by the
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``printk.always_kmsg_dump`` boot param: if unset, it'll be KMSG_DUMP_OOPS,
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otherwise KMSG_DUMP_MAX.
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Configurations for driver
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-------------------------
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Only a block device driver cares about these configurations. A block device
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driver uses ``register_pstore_blk`` to register to pstore/blk.
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A non-block device driver uses ``register_pstore_device`` with
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``struct pstore_device_info`` to register to pstore/blk.
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.. kernel-doc:: fs/pstore/blk.c
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:export:
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Compression and header
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----------------------
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Block device is large enough for uncompressed oops data. Actually we do not
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recommend data compression because pstore/blk will insert some information into
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the first line of oops/panic data. For example::
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Panic: Total 16 times
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It means that it's OOPS|Panic for the 16th time since the first booting.
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Sometimes the number of occurrences of oops|panic since the first booting is
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important to judge whether the system is stable.
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The following line is inserted by pstore filesystem. For example::
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Oops#2 Part1
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It means that it's OOPS for the 2nd time on the last boot.
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Reading the data
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----------------
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The dump data can be read from the pstore filesystem. The format for these
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files is ``dmesg-pstore-blk-[N]`` for oops/panic front-end,
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``pmsg-pstore-blk-0`` for pmsg front-end and so on. The timestamp of the
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dump file records the trigger time. To delete a stored record from block
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device, simply unlink the respective pstore file.
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Attentions in panic read/write APIs
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-----------------------------------
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If on panic, the kernel is not going to run for much longer, the tasks will not
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be scheduled and most kernel resources will be out of service. It
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looks like a single-threaded program running on a single-core computer.
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The following points require special attention for panic read/write APIs:
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1. Can **NOT** allocate any memory.
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If you need memory, just allocate while the block driver is initializing
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rather than waiting until the panic.
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#. Must be polled, **NOT** interrupt driven.
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No task schedule any more. The block driver should delay to ensure the write
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succeeds, but NOT sleep.
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#. Can **NOT** take any lock.
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There is no other task, nor any shared resource; you are safe to break all
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locks.
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#. Just use CPU to transfer.
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Do not use DMA to transfer unless you are sure that DMA will not keep lock.
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#. Control registers directly.
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Please control registers directly rather than use Linux kernel resources.
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Do I/O map while initializing rather than wait until a panic occurs.
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#. Reset your block device and controller if necessary.
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If you are not sure of the state of your block device and controller when
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a panic occurs, you are safe to stop and reset them.
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pstore/blk supports psblk_blkdev_info(), which is defined in
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*linux/pstore_blk.h*, to get information of using block device, such as the
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device number, sector count and start sector of the whole disk.
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pstore block internals
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----------------------
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For developer reference, here are all the important structures and APIs:
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.. kernel-doc:: fs/pstore/zone.c
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:internal:
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.. kernel-doc:: include/linux/pstore_zone.h
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:internal:
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.. kernel-doc:: fs/pstore/blk.c
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:internal:
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.. kernel-doc:: include/linux/pstore_blk.h
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:internal:
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