Re: [PATCH v6 21/21] docs: Document the FAN_FS_ERROR event
From: Jan Kara <jack@suse.cz>
Date: 2021-08-16 16:40:55
Also in:
linux-api, linux-fsdevel
On Thu 12-08-21 17:40:10, 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 <redacted>
<snip>
quoted hunk ↗ jump to hunk
diff --git a/Documentation/admin-guide/filesystem-monitoring.rst b/Documentation/admin-guide/filesystem-monitoring.rst new file mode 100644 index 000000000000..b03093567a93 --- /dev/null +++ b/Documentation/admin-guide/filesystem-monitoring.rst@@ -0,0 +1,70 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==================================== +File system Monitoring with fanotify +==================================== + +File system Error Reporting +=========================== + +fanotify supports the FAN_FS_ERROR mark for file system-wide error
^ Capital 'F'. ^^^ I'd rather write "event type".
+reporting. It is meant to be used by file system health monitoring +daemons who listen on that interface and take actions (notify sysadmin,
^^^ which ^^^^^^^^^^^^^^^^^ for these events
+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
^^^ additional
+errors. This ensures that the most important piece of error information
+is 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 user space example code is provided at ``samples/fanotify/fs-monitor.c``.
+
+A FAN_FS_ERROR Notification has the following format::
+
+ [ Notification Metadata (Mandatory) ]
+ [ Generic Error Record (Mandatory) ]
+ [ FID 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;
+ __s32 error;
+ __u32 error_count;
+ };
+
+The `error` field identifies the type of error. `error_count` count
+tracks the number of errors that occurred and were suppressed to
+preserve the original error, since the last notification.So is 'error' expected to be errno? Or is that some fs-specific error identifier? Will it be positive (i.e. real errno) or negative (as errno is usually passed in the kernel)? I think it should be specified here. Honza -- Jan Kara [off-list ref] SUSE Labs, CR