linux-ext4 - Re: [PATCH v6 21/21] docs: Document the FAN_FS

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Date:   Mon, 16 Aug 2021 18:40:43 +0200
From:   Jan Kara <jack@...e.cz>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     amir73il@...il.com, jack@...e.com, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        khazhy@...gle.com, dhowells@...hat.com, david@...morbit.com,
        tytso@....edu, djwong@...nel.org, repnop@...gle.com,
        kernel@...labora.com
Subject: Re: [PATCH v6 21/21] docs: Document the FAN_FS_ERROR event

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@...il.com>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>

<snip>

> 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 <jack@...e.com>
SUSE Labs, CR