ext4: avoid updating the superblock on a r/o mount if not needed

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commit 2ef6c32a91 upstream.

This was noticed by a user who noticied that the mtime of a file
backing a loopback device was getting bumped when the loopback device
is mounted read/only.  Note: This doesn't show up when doing a
loopback mount of a file directly, via "mount -o ro /tmp/foo.img
/mnt", since the loop device is set read-only when mount automatically
creates loop device.  However, this is noticeable for a LUKS loop
device like this:

% cryptsetup luksOpen /tmp/foo.img test
% mount -o ro /dev/loop0 /mnt ; umount /mnt

or, if LUKS is not in use, if the user manually creates the loop
device like this:

% losetup /dev/loop0 /tmp/foo.img
% mount -o ro /dev/loop0 /mnt ; umount /mnt

The modified mtime causes rsync to do a rolling checksum scan of the
file on the local and remote side, incrementally increasing the time
to rsync the not-modified-but-touched image file.

Fixes: eee00237fa ("ext4: commit super block if fs record error when journal record without error")
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/ZIauBR7YiV3rVAHL@glitch
Reported-by: Sean Greenslade <sean@seangreenslade.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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This commit is contained in:

Theodore Ts'o 2023-06-23 10:18:51 -04:00 • committed by Greg Kroah-Hartman

parent 08bd62023f

commit 03eb7f3001

1 changed files with 10 additions and 2 deletions

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fs/ext4/super.c

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