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Subject[PATCH v3 07/11] Documentation: create document for pstore/blk
Pstore/blk is a new back-end of pstore to dump oops/pmsg log to
block device.

The document, at Documentation/admin-guide/pstore-block.rst, tells us
how to use pstore/blk.

Signed-off-by: WeiXiong Liao <>
Documentation/admin-guide/pstore-block.rst | 223 +++++++++++++++++++++++++++++
fs/pstore/Kconfig | 2 +
3 files changed, 226 insertions(+)
create mode 100644 Documentation/admin-guide/pstore-block.rst

diff --git a/Documentation/admin-guide/pstore-block.rst b/Documentation/admin-guide/pstore-block.rst
new file mode 100644
index 000000000000..a96415eaaf24
--- /dev/null
+++ b/Documentation/admin-guide/pstore-block.rst
@@ -0,0 +1,223 @@
+.. SPDX-License-Identifier: GPL-2.0
+Pstore block oops/panic logger
+Pstore block (pstore/blk) is an oops/panic logger that writes its logs to a
+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,
+oops_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, 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=179:7 pstore_blk.oops_size=64
+The detail of each configurations may be of interest to you.
+The block device to use. Most of the time, it is a partition of block device.
+It's required for pstore/blk.
+It accepts the following variants:
+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.
+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.
+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
+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*.
+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
+Dumping both oopses and panics can be done by setting 1 (not zero) in the
+``dump_oops`` member while setting 0 in that variable dumps only the panics.
+Configurations for driver
+Only a block device driver cares about these configurations. A block device
+driver uses ``psblk_register_blkdev`` to register to pstore/blk.
+.. kernel-doc:: fs/pstore/pstore_blk.c
+ :identifiers: psblk_register_blkdev
+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/pstore_zone.c
+ :internal:
+.. kernel-doc:: include/linux/pstore_zone.h
+ :internal:
+.. kernel-doc:: fs/pstore/pstore_blk.c
+ :export:
+.. kernel-doc:: include/linux/pstore_blk.h
+ :internal:
index cc0a4a8ae06a..f553323b68c9 100644
@@ -13380,6 +13380,7 @@ F: include/linux/pstore*
F: drivers/firmware/efi/efi-pstore.c
F: drivers/acpi/apei/erst.c
F: Documentation/admin-guide/ramoops.rst
+F: Documentation/admin-guide/pstore-block.rst
F: Documentation/devicetree/bindings/reserved-memory/ramoops.txt
K: \b(pstore|ramoops)

diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index 55ce726be795..699f1925948f 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -171,6 +171,8 @@ config PSTORE_BLK
This enables panic and oops message to be logged to a block dev
where it can be read back at some later point.

+ For more information, see Documentation/admin-guide/pstore-block.rst.
If unsure, say N.

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