linux-ext4 - [Bug 214927] re-mount read-write (mount -oremount,rw) of read-only filesystem rejected with EROFS, but block device is not read-only

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Message-ID: <bug-214927-13602-0kDc4tAxpa@https.bugzilla.kernel.org/>
Date:   Wed, 03 Nov 2021 15:13:35 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...r.kernel.org
Subject: [Bug 214927] re-mount read-write (mount -oremount,rw) of read-only
 filesystem rejected with EROFS, but block device is not read-only

https://bugzilla.kernel.org/show_bug.cgi?id=214927

Theodore Tso (tytso@....edu) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tytso@....edu

--- Comment #3 from Theodore Tso (tytso@....edu) ---
In the case when there is a I/O error while trying to write to the journal,
there's nothing that can be done safely other than to force the file system to
be read-only.

When there is a file system which has aborted or otherwise has run into errors,
you have to unmount the file system before it is safe to remount it read/write.
 In fact, the ideal procedure is to umount the file system, run fsck, and then
remount the file system.

In the case of the root file system, you can't unmount the file system, so it
is acceptable to remount it read/only (if that hasn't been done automatically),
run fsck, and then reboot.

The reason for this because while the file system is mounted, there may be file
system corruption which was fixed by fsck, but for which some corruption (for
example, a corrupted refcount) is still present in memory.  So it is not safe
to take a mounted root file system, and modify it using fsck, and then remount
it read/write.   You have to reboot so it can be freshly mounted on the reboot.

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