Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+OCFS2 file system - online file check
+=====================================
+
+This document will describe OCFS2 online file check feature.
+
+Introduction
+============
+OCFS2 is often used in high-availability systems. However, OCFS2 usually
+converts the filesystem to read-only when encounters an error. This may not be
+necessary, since turning the filesystem read-only would affect other running
+processes as well, decreasing availability.
+Then, a mount option (errors=continue) is introduced, which would return the
+-EIO errno to the calling process and terminate further processing so that the
+filesystem is not corrupted further. The filesystem is not converted to
+read-only, and the problematic file's inode number is reported in the kernel
+log. The user can try to check/fix this file via online filecheck feature.
+
+Scope
+=====
+This effort is to check/fix small issues which may hinder day-to-day operations
+of a cluster filesystem by turning the filesystem read-only. The scope of
+checking/fixing is at the file level, initially for regular files and eventually
+to all files (including system files) of the filesystem.
+
+In case of directory to file links is incorrect, the directory inode is
+reported as erroneous.
+
+This feature is not suited for extravagant checks which involve dependency of
+other components of the filesystem, such as but not limited to, checking if the
+bits for file blocks in the allocation has been set. In case of such an error,
+the offline fsck should/would be recommended.
+
+Finally, such an operation/feature should not be automated lest the filesystem
+may end up with more damage than before the repair attempt. So, this has to
+be performed using user interaction and consent.
+
+User interface
+==============
+When there are errors in the OCFS2 filesystem, they are usually accompanied
+by the inode number which caused the error. This inode number would be the
+input to check/fix the file.
+
+There is a sysfs directory for each OCFS2 file system mounting::
+
+  /sys/fs/ocfs2/<devname>/filecheck
+
+Here, <devname> indicates the name of OCFS2 volume device which has been already
+mounted. The file above would accept inode numbers. This could be used to
+communicate with kernel space, tell which file(inode number) will be checked or
+fixed. Currently, three operations are supported, which includes checking
+inode, fixing inode and setting the size of result record history.
+
+1. If you want to know what error exactly happened to <inode> before fixing, do::
+
+    # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/check
+    # cat /sys/fs/ocfs2/<devname>/filecheck/check
+
+The output is like this::
+
+    INO		DONE	ERROR
+    39502		1	GENERATION
+
+    <INO> lists the inode numbers.
+    <DONE> indicates whether the operation has been finished.
+    <ERROR> says what kind of errors was found. For the detailed error numbers,
+    please refer to the file linux/fs/ocfs2/filecheck.h.
+
+2. If you determine to fix this inode, do::
+
+    # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/fix
+    # cat /sys/fs/ocfs2/<devname>/filecheck/fix
+
+The output is like this:::
+
+    INO		DONE	ERROR
+    39502		1	SUCCESS
+
+This time, the <ERROR> column indicates whether this fix is successful or not.
+
+3. The record cache is used to store the history of check/fix results. It's
+default size is 10, and can be adjust between the range of 10 ~ 100. You can
+adjust the size like this::
+
+  # echo "<size>" > /sys/fs/ocfs2/<devname>/filecheck/set
+
+Fixing stuff
+============
+On receiving the inode, the filesystem would read the inode and the
+file metadata. In case of errors, the filesystem would fix the errors
+and report the problems it fixed in the kernel log. As a precautionary measure,
+the inode must first be checked for errors before performing a final fix.
+
+The inode and the result history will be maintained temporarily in a
+small linked list buffer which would contain the last (N) inodes
+fixed/checked, the detailed errors which were fixed/checked are printed in the
+kernel log.