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

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: eee0023 ("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>

Author: tytso

fs/ext4/super.c

@@ -5997,19 +5997,27 @@ static int ext4_load_journal(struct super_block *sb,
 		err = jbd2_journal_wipe(journal, !really_read_only);
 	if (!err) {
 		char *save = kmalloc(EXT4_S_ERR_LEN, GFP_KERNEL);
+		__le16 orig_state;
+		bool changed = false;
 
 		if (save)
 			memcpy(save, ((char *) es) +
 			       EXT4_S_ERR_START, EXT4_S_ERR_LEN);
 		err = jbd2_journal_load(journal);
-		if (save)
+		if (save && memcmp(((char *) es) + EXT4_S_ERR_START,
+				   save, EXT4_S_ERR_LEN)) {
 			memcpy(((char *) es) + EXT4_S_ERR_START,
 			       save, EXT4_S_ERR_LEN);
+			changed = true;
+		}
 		kfree(save);
+		orig_state = es->s_state;
 		es->s_state |= cpu_to_le16(EXT4_SB(sb)->s_mount_state &
 					   EXT4_ERROR_FS);
+		if (orig_state != es->s_state)
+			changed = true;
 		/* Write out restored error information to the superblock */
-		if (!bdev_read_only(sb->s_bdev)) {
+		if (changed && !really_read_only) {
 			int err2;
 			err2 = ext4_commit_super(sb);
 			err = err ? : err2;