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: eee00237fa5e ("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>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b3819e70093ea00521e5a8f51ef766941ced065a..c638b0db3b2bc9040b12e32f2b1c49f4f14aa3ae 100644 (file)
--- a/fs/ext4/super.c
+++ b/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;