1 .. SPDX-License-Identifier: GPL-2.0
2
3 ====================================
4 File system Monitoring with fanotify
5 ====================================
6
7 File system Error Reporting
8 ===========================
9
10 Fanotify supports the FAN_FS_ERROR event type for file system-wide error
11 reporting. It is meant to be used by file system health monitoring
12 daemons, which listen for these events and take actions (notify
13 sysadmin, start recovery) when a file system problem is detected.
14
15 By design, a FAN_FS_ERROR notification exposes sufficient information
16 for a monitoring tool to know a problem in the file system has happened.
17 It doesn't necessarily provide a user space application with semantics
18 to verify an IO operation was successfully executed. That is out of
19 scope for this feature. Instead, it is only meant as a framework for
20 early file system problem detection and reporting recovery tools.
21
22 When a file system operation fails, it is common for dozens of kernel
23 errors to cascade after the initial failure, hiding the original failure
24 log, which is usually the most useful debug data to troubleshoot the
25 problem. For this reason, FAN_FS_ERROR tries to report only the first
26 error that occurred for a file system since the last notification, and
27 it simply counts additional errors. This ensures that the most
28 important pieces of information are never lost.
29
30 FAN_FS_ERROR requires the fanotify group to be setup with the
31 FAN_REPORT_FID flag.
32
33 At the time of this writing, the only file system that emits FAN_FS_ERROR
34 notifications is Ext4.
35
36 A FAN_FS_ERROR Notification has the following format::
37
38 ::
39
40 [ Notification Metadata (Mandatory) ]
41 [ Generic Error Record (Mandatory) ]
42 [ FID record (Mandatory) ]
43
44 The order of records is not guaranteed, and new records might be added
45 in the future. Therefore, applications must not rely on the order and
46 must be prepared to skip over unknown records. Please refer to
47 ``samples/fanotify/fs-monitor.c`` for an example parser.
48
49 Generic error record
50 --------------------
51
52 The generic error record provides enough information for a file system
53 agnostic tool to learn about a problem in the file system, without
54 providing any additional details about the problem. This record is
55 identified by ``struct fanotify_event_info_header.info_type`` being set
56 to FAN_EVENT_INFO_TYPE_ERROR.
57
58 ::
59
60 struct fanotify_event_info_error {
61 struct fanotify_event_info_header hdr;
62 __s32 error;
63 __u32 error_count;
64 };
65
66 The `error` field identifies the type of error using errno values.
67 `error_count` tracks the number of errors that occurred and were
68 suppressed to preserve the original error information, since the last
69 notification.
70
71 FID record
72 ----------
73
74 The FID record can be used to uniquely identify the inode that triggered
75 the error through the combination of fsid and file handle. A file system
76 specific application can use that information to attempt a recovery
77 procedure. Errors that are not related to an inode are reported with an
78 empty file handle of type FILEID_INVALID.
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