On Tue, Jul 20, 2021 at 7:00 PM Gabriel Krisman Bertazi
<krisman@xxxxxxxxxxxxx> wrote:
>
> Document the FAN_FS_ERROR event for user administrators and user space
> developers.
>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
>
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Expect one outdated detail...
> ---
> Changes Since v3:
> - Move FAN_FS_ERROR notification into a subsection of the file.
> Changes Since v2:
> - NTR
> 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 | 70 +++++++++++++++++++
> Documentation/admin-guide/index.rst | 1 +
> 2 files changed, 71 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..6c8982985a27
> --- /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
> +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.
> +
> +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.
> +
> +FID record
> +----------
> +
> +The FID record can be used to uniquely identify the inode that triggered
> +the error through the combination of fsid and file handler. A
> +filesystem specific handler can use that information to attempt a
> +recovery procedure. Errors that are not related to an inode are
> +reported against the root inode.
Not uptodate...
Thanks,
Amir.
