btrfs: make sure SB_I_VERSION doesn't get unset by remount

commit faa008899a4db21a2df99833cb4ff6fa67009a20 upstream.

There's some inconsistency around SB_I_VERSION handling with mount and
remount.  Since we don't really want it to be off ever just work around
this by making sure we don't get the flag cleared on remount.

There's a tiny cpu cost of setting the bit, otherwise all changes to
i_version also change some of the times (ctime/mtime) so the inode needs
to be synced. We wouldn't save anything by disabling it.

Reported-by: Eric Sandeen <sandeen@redhat.com>
CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add perf impact analysis ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index f82b3a18b051dabcc5c4f5f3ffe394f258e16612..6ca9bc3f51be16e00151a7e81b1964851e20829c 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1895,6 +1895,12 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
                set_bit(BTRFS_FS_OPEN, &fs_info->flags);
        }
 out:
+       /*
+        * We need to set SB_I_VERSION here otherwise it'll get cleared by VFS,
+        * since the absence of the flag means it can be toggled off by remount.
+        */
+       *flags |= SB_I_VERSION;
+
        wake_up_process(fs_info->transaction_kthread);
        btrfs_remount_cleanup(fs_info, old_opts);
        return 0;