btrfs: bail out reclaim process if filesystem is read-only

When a filesystem is read-only, we cannot reclaim a block group as it
cannot rewrite the data. Just bail out in that case.

Note that it can drop block groups in this case. As we did
sb_start_write(), read-only filesystem means we got a fatal error and
forced read-only. There is no chance to reclaim them again.

Fixes: 18bb8bbf ("btrfs: zoned: automatically reclaim zones")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -1794,8 +1794,15 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 		}
 		spin_unlock(&bg->lock);
 
-		/* Get out fast, in case we're unmounting the filesystem */
-		if (btrfs_fs_closing(fs_info)) {
+		/*
+		 * Get out fast, in case we're read-only or unmounting the
+		 * filesystem. It is OK to drop block groups from the list even
+		 * for the read-only case. As we did sb_start_write(),
+		 * "mount -o remount,ro" won't happen and read-only filesystem
+		 * means it is forced read-only due to a fatal error. So, it
+		 * never gets back to read-write to let us reclaim again.
+		 */
+		if (btrfs_need_cleaner_sleep(fs_info)) {
 			up_write(&space_info->groups_sem);
 			goto next;
 		}