Documentation: Document the FAN_FS_ERROR event

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

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

---
Changes since v1:
  - Drop references to location record
  - Explain that the inode field is optional
  - Explain we are reporting only the first error

Documentation/admin-guide/filesystem-monitoring.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
+File system Monitoring with fanotify
+====================================
+
+fanotify supports the FAN_FS_ERROR mark for file system-wide error
+reporting.  It is meant to be used by file system health monitoring
+daemons who listen on that interface and take actions (notify sysadmin,
+start recovery) when a file system problem is detected by the kernel.
+
+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 outside of
+scope of 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 only reports the first error that
+occurred since the last notification, and it simply counts addition
+errors.  This ensures that the most important piece of error information
+is never lost.
+
+At the time of this writing, the only file system that emits FAN_FS_ERROR
+notifications is Ext4.
+
+A user space example code is provided at ``samples/fanotify/fs-monitor.c``.
+
+Notification structure
+======================
+
+A FAN_FS_ERROR Notification has the following format::
+
+  [ Notification Metadata (Mandatory) ]
+  [ Generic Error Record  (Mandatory) ]
+
+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;
+	int error;
+	__u32 error_count;
+	__kernel_fsid_t fsid;
+	__u64 ino;
+	__u32 ino_generation;
+  };
+
+The `error` field identifies the type of error.  `fsid` identifies the
+file system that originated the error, since multiple file systems might
+be reporting to the same fanotify group.  The `inode` field is optional,
+as it depends on whether the error is linked to an specific inode.
+`error_count` count tracks the number of errors that occurred and were
+suppressed to preserve the original error, since the last notification.

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