[PATCH v2 14/14] Documentation: Document the FAN_FS_ERROR event
From: Gabriel Krisman Bertazi <hidden>
Date: 2021-06-15 23:56:58
Also in:
linux-ext4
Subsystem:
documentation, the rest · Maintainers:
Jonathan Corbet, Linus Torvalds
Document the FAN_FS_ERROR event for user administrators and user space developers. Signed-off-by: Gabriel Krisman Bertazi <redacted> --- 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 | 63 +++++++++++++++++++ Documentation/admin-guide/index.rst | 1 + 2 files changed, 64 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..3710302676af
--- /dev/null
+++ b/Documentation/admin-guide/filesystem-monitoring.rst@@ -0,0 +1,63 @@ +.. 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.
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
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2.31.0