79 lines
3.0 KiB
ReStructuredText
79 lines
3.0 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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====================================
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File system Monitoring with fanotify
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====================================
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File system Error Reporting
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===========================
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Fanotify supports the FAN_FS_ERROR event type for file system-wide error
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reporting. It is meant to be used by file system health monitoring
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daemons, which listen for these events and take actions (notify
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sysadmin, start recovery) when a file system problem is detected.
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By design, a FAN_FS_ERROR notification exposes sufficient information
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for a monitoring tool to know a problem in the file system has happened.
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It doesn't necessarily provide a user space application with semantics
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to verify an IO operation was successfully executed. That is out of
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scope for this feature. Instead, it is only meant as a framework for
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early file system problem detection and reporting recovery tools.
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When a file system operation fails, it is common for dozens of kernel
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errors to cascade after the initial failure, hiding the original failure
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log, which is usually the most useful debug data to troubleshoot the
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problem. For this reason, FAN_FS_ERROR tries to report only the first
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error that occurred for a file system since the last notification, and
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it simply counts additional errors. This ensures that the most
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important pieces of information are never lost.
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FAN_FS_ERROR requires the fanotify group to be setup with the
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FAN_REPORT_FID flag.
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At the time of this writing, the only file system that emits FAN_FS_ERROR
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notifications is Ext4.
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A FAN_FS_ERROR Notification has the following format::
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::
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[ Notification Metadata (Mandatory) ]
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[ Generic Error Record (Mandatory) ]
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[ FID record (Mandatory) ]
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The order of records is not guaranteed, and new records might be added
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in the future. Therefore, applications must not rely on the order and
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must be prepared to skip over unknown records. Please refer to
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``samples/fanotify/fs-monitor.c`` for an example parser.
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Generic error record
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--------------------
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The generic error record provides enough information for a file system
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agnostic tool to learn about a problem in the file system, without
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providing any additional details about the problem. This record is
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identified by ``struct fanotify_event_info_header.info_type`` being set
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to FAN_EVENT_INFO_TYPE_ERROR.
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::
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struct fanotify_event_info_error {
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struct fanotify_event_info_header hdr;
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__s32 error;
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__u32 error_count;
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};
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The `error` field identifies the type of error using errno values.
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`error_count` tracks the number of errors that occurred and were
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suppressed to preserve the original error information, since the last
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notification.
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FID record
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----------
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The FID record can be used to uniquely identify the inode that triggered
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the error through the combination of fsid and file handle. A file system
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specific application can use that information to attempt a recovery
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procedure. Errors that are not related to an inode are reported with an
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empty file handle of type FILEID_INVALID.
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