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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 | ||
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38 | :: |
39 | ||
40 | [ Notification Metadata (Mandatory) ] | |
41 | [ Generic Error Record (Mandatory) ] | |
42 | [ FID record (Mandatory) ] | |
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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 | ||
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58 | :: |
59 | ||
60 | struct fanotify_event_info_error { | |
61 | struct fanotify_event_info_header hdr; | |
62 | __s32 error; | |
63 | __u32 error_count; | |
64 | }; | |
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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. |