dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure

blk-mq will rerun queue via RESTART or dispatch wake after one request
is completed, so not necessary to wait random time for requeuing, we
should trust blk-mq to do it.

More importantly, we need to return BLK_STS_RESOURCE to blk-mq so that
dequeuing from the I/O scheduler can be stopped, this results in
improved I/O merging.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -533,8 +533,20 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
 		if (blk_queue_dying(q)) {
 			atomic_inc(&m->pg_init_in_progress);
 			activate_or_offline_path(pgpath);
+			return DM_MAPIO_DELAY_REQUEUE;
 		}
-		return DM_MAPIO_DELAY_REQUEUE;
+
+		/*
+		 * blk-mq's SCHED_RESTART can cover this requeue, so we
+		 * needn't deal with it by DELAY_REQUEUE. More importantly,
+		 * we have to return DM_MAPIO_REQUEUE so that blk-mq can
+		 * get the queue busy feedback (via BLK_STS_RESOURCE),
+		 * otherwise I/O merging can suffer.
+		 */
+		if (q->mq_ops)
+			return DM_MAPIO_REQUEUE;
+		else
+			return DM_MAPIO_DELAY_REQUEUE;
 	}
 	clone->bio = clone->biotail = NULL;
 	clone->rq_disk = bdev->bd_disk;