On 11/06/2025 15:53, Jens Axboe wrote:
CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe. 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@xxxxxxxxxxxxxxx Link: https://lore.kernel.org/linux-block/20250611121626.7252-1-abuehaze@xxxxxxxxxx/ Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@xxxxxxxxxx> Fixes: e70c301faece ("block: don't reorder requests in blk_add_rq_to_plug") Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> --- diff --git a/block/blk-merge.c b/block/blk-merge.c index 3af1d284add5..70d704615be5 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -998,20 +998,20 @@ 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; } -- Jens Axboe
Thanks for posting this fix, I can confirm that this patch mitigated the regression reported in [1], my only concern is the impact when we have multiple queues available as you explained in the commit message. Given that reverting e70c301faece ("block: don't reorder requests in blk_add_rq_to_plug") will break zoned storage support and the plug list is singly linked, I don't think we have a better solution here.
[1] https://lore.kernel.org/lkml/20250611121626.7252-1-abuehaze@xxxxxxxxxx/T/#u
Hazem
