[v9,31/31] docs: Document the FAN_FS_ERROR event

Document the FAN_FS_ERROR event for user administrators and user space
developers.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

---
Changes Since v8:
  - Replace fs-error specific errno bits with generic errno. (Jan)
  - Explain event order guarantees and point to example parser (Jan)
Changes Since v7:
  - Update semantics
Changes Since v6:
  - English fixes (jan)
  - Proper document error field (jan)
Changes Since v4:
  - Update documentation about reporting non-file error.
Changes Since v3:
  - Move FAN_FS_ERROR notification into a subsection of the file.
Changes Since v2:
  - NTR
Changes since v1:
  - Drop references to location record
  - Explain that the inode field is optional
  - Explain we are reporting only the first error
---
 .../admin-guide/filesystem-monitoring.rst     | 74 +++++++++++++++++++
 Documentation/admin-guide/index.rst           |  1 +
 2 files changed, 75 insertions(+)
 create mode 100644 Documentation/admin-guide/filesystem-monitoring.rst

On Mon 25-10-21 16:27:46, Gabriel Krisman Bertazi wrote:
> Document the FAN_FS_ERROR event for user administrators and user space
> developers.
> 
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> 
> ---
> Changes Since v8:
>   - Replace fs-error specific errno bits with generic errno. (Jan)
>   - Explain event order guarantees and point to example parser (Jan)
> Changes Since v7:
>   - Update semantics
> Changes Since v6:
>   - English fixes (jan)
>   - Proper document error field (jan)
> Changes Since v4:
>   - Update documentation about reporting non-file error.
> Changes Since v3:
>   - Move FAN_FS_ERROR notification into a subsection of the file.
> Changes Since v2:
>   - NTR
> Changes since v1:
>   - Drop references to location record
>   - Explain that the inode field is optional
>   - Explain we are reporting only the first error
> ---
>  .../admin-guide/filesystem-monitoring.rst     | 74 +++++++++++++++++++
>  Documentation/admin-guide/index.rst           |  1 +
>  2 files changed, 75 insertions(+)
>  create mode 100644 Documentation/admin-guide/filesystem-monitoring.rst
> 
> diff --git a/Documentation/admin-guide/filesystem-monitoring.rst b/Documentation/admin-guide/filesystem-monitoring.rst
> new file mode 100644
> index 000000000000..5a3c84e60095
> --- /dev/null
> +++ b/Documentation/admin-guide/filesystem-monitoring.rst
> @@ -0,0 +1,74 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +====================================
> +File system Monitoring with fanotify
> +====================================
> +
> +File system Error Reporting
> +===========================
> +
> +Fanotify supports the FAN_FS_ERROR event type for file system-wide error
> +reporting.  It is meant to be used by file system health monitoring
> +daemons, which listen for these events and take actions (notify
> +sysadmin, start recovery) when a file system problem is detected.
> +
> +By design, a FAN_FS_ERROR notification exposes sufficient information
> +for a monitoring tool to know a problem in the file system has happened.
> +It doesn't necessarily provide a user space application with semantics
> +to verify an IO operation was successfully executed.  That is out of
> +scope for this feature.  Instead, it is only meant as a framework for
> +early file system problem detection and reporting recovery tools.
> +
> +When a file system operation fails, it is common for dozens of kernel
> +errors to cascade after the initial failure, hiding the original failure
> +log, which is usually the most useful debug data to troubleshoot the
> +problem.  For this reason, FAN_FS_ERROR tries to report only the first
> +error that occurred for a file system since the last notification, and
> +it simply counts additional errors.  This ensures that the most
> +important pieces of information are never lost.
> +
> +FAN_FS_ERROR requires the fanotify group to be setup with the
> +FAN_REPORT_FID flag.
> +
> +At the time of this writing, the only file system that emits FAN_FS_ERROR
> +notifications is Ext4.
> +
> +A FAN_FS_ERROR Notification has the following format::
> +
> +  [ Notification Metadata (Mandatory) ]
> +  [ Generic Error Record  (Mandatory) ]
> +  [ FID record            (Mandatory) ]
> +
> +The order of records is not guaranteed, and new records might be added
> +in the future.  Therefore, applications must not rely on the order and
> +must be prepared to skip over unknown records. Please refer to
> +``samples/fanotify/fs-monitor.c`` for an example parser.
> +
> +Generic error record
> +--------------------
> +
> +The generic error record provides enough information for a file system
> +agnostic tool to learn about a problem in the file system, without
> +providing any additional details about the problem.  This record is
> +identified by ``struct fanotify_event_info_header.info_type`` being set
> +to FAN_EVENT_INFO_TYPE_ERROR.
> +
> +  struct fanotify_event_info_error {
> +	struct fanotify_event_info_header hdr;
> +	__s32 error;
> +	__u32 error_count;
> +  };
> +
> +The `error` field identifies the type of error using errno values.
> +`error_count` tracks the number of errors that occurred and were
> +suppressed to preserve the original error information, since the last
> +notification.
> +
> +FID record
> +----------
> +
> +The FID record can be used to uniquely identify the inode that triggered
> +the error through the combination of fsid and file handle.  A file system
> +specific application can use that information to attempt a recovery
> +procedure.  Errors that are not related to an inode are reported with an
> +empty file handle of type FILEID_INVALID.
> diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
> index dc00afcabb95..1bedab498104 100644
> --- a/Documentation/admin-guide/index.rst
> +++ b/Documentation/admin-guide/index.rst
> @@ -82,6 +82,7 @@ configure specific aspects of kernel behavior to your liking.
>     edid
>     efi-stub
>     ext4
> +   filesystem-monitoring
>     nfs/index
>     gpio/index
>     highuid
> -- 
> 2.33.0
>