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Date:   Sun, 10 May 2020 13:24:31 -0700
From:   Kees Cook <>
To:     WeiXiong Liao <>
Cc:     Kees Cook <>,
        Anton Vorontsov <>,
        Colin Cross <>,
        Tony Luck <>,
        Jonathan Corbet <>,
        Miquel Raynal <>,
        Richard Weinberger <>,
        Vignesh Raghavendra <>,
        Petr Mladek <>,
        Sergey Senozhatsky <>,
        Steven Rostedt <>,
        Rob Herring <>,
        Pavel Tatashin <>,,,
Subject: [PATCH v7 13/18] Documentation: Add details for pstore/blk

From: WeiXiong Liao <>

Add details on using pstore/blk, the new backend of pstore to record
dumps to block devices, in Documentation/admin-guide/pstore-blk.rst

Signed-off-by: WeiXiong Liao <>
Signed-off-by: Kees Cook <>
 Documentation/admin-guide/pstore-blk.rst | 229 +++++++++++++++++++++++
 MAINTAINERS                              |   1 +
 fs/pstore/Kconfig                        |   2 +
 3 files changed, 232 insertions(+)
 create mode 100644 Documentation/admin-guide/pstore-blk.rst

diff --git a/Documentation/admin-guide/pstore-blk.rst b/Documentation/admin-guide/pstore-blk.rst
new file mode 100644
index 000000000000..bef8c7436721
--- /dev/null
+++ b/Documentation/admin-guide/pstore-blk.rst
@@ -0,0 +1,229 @@
+.. 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,
+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, 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.kmsg_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
+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
+Only a block device driver cares about these configurations. A block device
+driver uses ``register_pstore_blk`` to register to pstore/blk.
+.. kernel-doc:: fs/pstore/blk.c
+   :identifiers: register_pstore_blk
+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
+   :export:
+.. kernel-doc:: include/linux/pstore_blk.h
+   :internal:
index e64e5db31497..9c1f4feff418 100644
@@ -13660,6 +13660,7 @@ M:	Tony Luck <>
 S:	Maintained
 T:	git git:// for-next/pstore
 F:	Documentation/admin-guide/ramoops.rst
+F:	Documentation/admin-guide/pstore-blk.rst
 F:	Documentation/devicetree/bindings/reserved-memory/ramoops.txt
 F:	drivers/acpi/apei/erst.c
 F:	drivers/firmware/efi/efi-pstore.c
diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index 16a0440d8d5a..8371d29651a6 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-blk.rst
 	  If unsure, say N.

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