btrfs: log message when rw remount is attempted with unclean tree-log

A remount to a read-write filesystem is not safe when there's tree-log
to be replayed. Files that could be opened until now might be affected
by the changes in the tree-log.

A regular mount is needed to replay the log so the filesystem presents
the consistent view with the pending changes included.

CC: [email protected] # 4.4+
Reviewed-by: Anand Jain <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: kdave

fs/btrfs/super.c

@@ -1834,6 +1834,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		}
 
 		if (btrfs_super_log_root(fs_info->super_copy) != 0) {
+			btrfs_warn(fs_info,
+		"mount required to replay tree-log, cannot remount read-write");
 			ret = -EINVAL;
 			goto restore;
 		}