pstore block oops/panic logger — The Linux Kernel 5.18.5-gnu-1 documentation

# 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

2. /dev/<disk_name><decimal> represents the device number of partition - device number of disk plus the partition number

3. /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:

1. <hex_major><hex_minor> device number in hexadecimal representation, with no leading 0x, for example b302.

2. 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.

3. PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to a partition with a known unique id.

4. <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.

2. <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.

int register_pstore_device(struct pstore_device_info *dev)

  register non-block device to pstore/blk

Parameters

`struct pstore_device_info *dev`

  non-block device information

Return

- 0 - OK

- Others - something error.

void unregister_pstore_device(struct pstore_device_info *dev)

  unregister non-block device from pstore/blk

Parameters

`struct pstore_device_info *dev`

  non-block device information

## 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.

2. Must be polled, NOT interrupt driven. No task schedule any more. The block driver should delay to ensure the write succeeds, but NOT sleep.

3. Can NOT take any lock. There is no other task, nor any shared resource; you are safe to break all locks.

4. Just use CPU to transfer. Do not use DMA to transfer unless you are sure that DMA will not keep lock.

5. 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.

6. 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:

struct psz_buffer

  header of zone to flush to storage

Definition

    struct psz_buffer {
    #define PSZ_SIG (0x43474244) ;
      uint32_t sig;
      atomic_t datalen;
      atomic_t start;
      uint8_t data[];
    };

Members

`sig`

  signature to indicate header (PSZ_SIG xor PSZONE-type value)

`datalen`

  length of data in data

`start`

  offset into data where the beginning of the stored bytes begin

`data`

  zone data.

struct psz_kmsg_header

  kmsg dump-specific header to flush to storage

Definition

    struct psz_kmsg_header {
    #define PSTORE_KMSG_HEADER_MAGIC 0x4dfc3ae5 ;
      uint32_t magic;
      struct timespec64 time;
      bool compressed;
      uint32_t counter;
      enum kmsg_dump_reason reason;
      uint8_t data[];
    };

Members

`magic`

  magic num for kmsg dump header

`time`

  kmsg dump trigger time

`compressed`

  whether conpressed

`counter`

  kmsg dump counter

`reason`

  the kmsg dump reason (e.g. oops, panic, etc)

`data`

  pointer to log data

Description

This is a sub-header for a kmsg dump, trailing after `psz_buffer`.

struct pstore_zone

  single stored buffer

Definition

    struct pstore_zone {
      loff_t off;
      const char *name;
      enum pstore_type_id type;
      struct psz_buffer *buffer;
      struct psz_buffer *oldbuf;
      size_t buffer_size;
      bool should_recover;
      atomic_t dirty;
    };

Members

`off`

  zone offset of storage

`name`

  front-end name for this zone

`type`

  front-end type for this zone

`buffer`

  pointer to data buffer managed by this zone

`oldbuf`

  pointer to old data buffer

`buffer_size`

  bytes in buffer->data

`should_recover`

  whether this zone should recover from storage

`dirty`

  whether the data in buffer dirty

Description

zone structure in memory.

struct psz_context

  all about running state of pstore/zone

Definition

    struct psz_context {
      struct pstore_zone **kpszs;
      struct pstore_zone *ppsz;
      struct pstore_zone *cpsz;
      struct pstore_zone **fpszs;
      unsigned int kmsg_max_cnt;
      unsigned int kmsg_read_cnt;
      unsigned int kmsg_write_cnt;
      unsigned int pmsg_read_cnt;
      unsigned int console_read_cnt;
      unsigned int ftrace_max_cnt;
      unsigned int ftrace_read_cnt;
      unsigned int oops_counter;
      unsigned int panic_counter;
      atomic_t recovered;
      atomic_t on_panic;
      struct mutex pstore_zone_info_lock;
      struct pstore_zone_info *pstore_zone_info;
      struct pstore_info pstore;
    };

Members

`kpszs`

  kmsg dump storage zones

`ppsz`

  pmsg storage zone

`cpsz`

  console storage zone

`fpszs`

  ftrace storage zones

`kmsg_max_cnt`

  max count of kpszs

`kmsg_read_cnt`

  counter of total read kmsg dumps

`kmsg_write_cnt`

  counter of total kmsg dump writes

`pmsg_read_cnt`

  counter of total read pmsg zone

`console_read_cnt`

  counter of total read console zone

`ftrace_max_cnt`

  max count of fpszs

`ftrace_read_cnt`

  counter of max read ftrace zone

`oops_counter`

  counter of oops dumps

`panic_counter`

  counter of panic dumps

`recovered`

  whether finished recovering data from storage

`on_panic`

  whether panic is happening

`pstore_zone_info_lock`

  lock to pstore_zone_info

`pstore_zone_info`

  information from backend

`pstore`

  structure for pstore

enum psz_flush_mode

  flush mode for psz_zone_write()

Constants

`FLUSH_NONE`

  do not flush to storage but update data on memory

`FLUSH_PART`

  just flush part of data including meta data to storage

`FLUSH_META`

  just flush meta data of zone to storage

`FLUSH_ALL`

  flush all of zone

int psz_recovery(struct psz_context *cxt)

  recover data from storage

Parameters

`struct psz_context *cxt`

  the context of pstore/zone

Description

recovery means reading data back from storage after rebooting

Return

0 on success, others on failure.

struct pstore_zone_info

  pstore/zone back-end driver structure

Definition

    struct pstore_zone_info {
      struct module *owner;
      const char *name;
      unsigned long total_size;
      unsigned long kmsg_size;
      int max_reason;
      unsigned long pmsg_size;
      unsigned long console_size;
      unsigned long ftrace_size;
      pstore_zone_read_op read;
      pstore_zone_write_op write;
      pstore_zone_erase_op erase;
      pstore_zone_write_op panic_write;
    };

Members

`owner`

  Module which is responsible for this back-end driver.

`name`

  Name of the back-end driver.

`total_size`

  The total size in bytes pstore/zone can use. It must be greater than 4096 and be multiple of 4096.

`kmsg_size`

  The size of oops/panic zone. Zero means disabled, otherwise, it must be multiple of SECTOR_SIZE(512 Bytes).

`max_reason`

  Maximum kmsg dump reason to store.

`pmsg_size`

  The size of pmsg zone which is the same as kmsg_size.

`console_size`

  The size of console zone which is the same as kmsg_size.

`ftrace_size`

  The size of ftrace zone which is the same as kmsg_size.

`read`

  The general read operation. Both of the function parameters size and offset are relative value to storage. On success, the number of bytes should be returned, others mean error.

`write`

  The same as read, but the following error number: -EBUSY means try to write again later. -ENOMSG means to try next zone.

`erase`

  The general erase operation for device with special removing job. Both of the function parameters size and offset are relative value to storage. Return 0 on success and others on failure.

`panic_write`

  The write operation only used for panic case. It’s optional if you do not care panic log. The parameters are relative value to storage. On success, the number of bytes should be returned, others excluding -ENOMSG mean error. -ENOMSG means to try next zone.

struct pstore_device_info

  back-end pstore/blk driver structure.

Definition

    struct pstore_device_info {
      unsigned int flags;
      struct pstore_zone_info zone;
    };

Members

`flags`

  Refer to macro starting with PSTORE_FLAGS defined in linux/pstore.h. It means what front-ends this device support. Zero means all backends for compatible.

`zone`

  The `struct pstore_zone_info` details.

struct pstore_blk_config

  the pstore_blk backend configuration

Definition

    struct pstore_blk_config {
      char device[80];
      enum kmsg_dump_reason max_reason;
      unsigned long kmsg_size;
      unsigned long pmsg_size;
      unsigned long console_size;
      unsigned long ftrace_size;
    };

Members

`device`

  Name of the desired block device

`max_reason`

  Maximum kmsg dump reason to store to block device

`kmsg_size`

  Total size of for kmsg dumps

`pmsg_size`

  Total size of the pmsg storage area

`console_size`

  Total size of the console storage area

`ftrace_size`

  Total size for ftrace logging data (for all CPUs)

int pstore_blk_get_config(struct pstore_blk_config *info)

  get a copy of the pstore_blk backend configuration

Parameters

`struct pstore_blk_config *info`

  The sturct pstore_blk_config to be filled in

Description

Failure returns negative error code, and success returns 0.