block: use plug request list tail for one-shot backmerge attempt

JIRA: https://issues.redhat.com/browse/RHEL-96294

commit 961296e
Author: Jens Axboe <axboe@kernel.dk>
Date:   Wed Jun 11 08:48:46 2025 -0600

    block: use plug request list tail for one-shot backmerge attempt

    Previously, the block layer stored the requests in the plug list in
    LIFO order. For this reason, blk_attempt_plug_merge() would check
    just the head entry for a back merge attempt, and abort after that
    unless requests for multiple queues existed in the plug list. If more
    than one request is present in the plug list, this makes the one-shot
    back merging less useful than before, as it'll always fail to find a
    quick merge candidate.

    Use the tail entry for the one-shot merge attempt, which is the last
    added request in the list. If that fails, abort immediately unless
    there are multiple queues available. If multiple queues are available,
    then scan the list. Ideally the latter scan would be a backwards scan
    of the list, but as it currently stands, the plug list is singly linked
    and hence this isn't easily feasible.

    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/linux-block/20250611121626.7252-1-abuehaze@amazon.com/
    Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@amazon.com>
    Fixes: e70c301 ("block: don't reorder requests in blk_add_rq_to_plug")
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-merge.c

@@ -1127,20 +1127,20 @@ bool blk_attempt_plug_merge(struct request_queue *q, struct bio *bio,
 	if (!plug || rq_list_empty(&plug->mq_list))
 		return false;
 
-	rq_list_for_each(&plug->mq_list, rq) {
-		if (rq->q == q) {
-			if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
-			    BIO_MERGE_OK)
-				return true;
-			break;
-		}
+	rq = plug->mq_list.tail;
+	if (rq->q == q)
+		return blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
+			BIO_MERGE_OK;
+	else if (!plug->multiple_queues)
+		return false;
 
-		/*
-		 * Only keep iterating plug list for merges if we have multiple
-		 * queues
-		 */
-		if (!plug->multiple_queues)
-			break;
+	rq_list_for_each(&plug->mq_list, rq) {
+		if (rq->q != q)
+			continue;
+		if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==
+		    BIO_MERGE_OK)
+			return true;
+		break;
 	}
 	return false;
 }