Re: [PATCH] Revert "block: don't reorder requests in blk_add_rq_to_plug"

[Jens Axboe]

On 6/11/25 6:14 AM, Hazem Mohamed Abuelfotoh wrote:
> This reverts commit e70c301faece15b618e54b613b1fd6ece3dd05b4.
> 
> Commit <e70c301faece> ("block: don't reorder requests in
> blk_add_rq_to_plug") reversed how requests are stored in the blk_plug
> list, this had significant impact on bio merging with requests exist on
> the plug list. This impact has been reported in [1] and could easily be
> reproducible using 4k randwrite fio benchmark on an NVME based SSD without
> having any filesystem on the disk.

Rather than revert this commit, why not just attempt a tail merge?
Something ala this, totally untested.

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 3af1d284add5..708ded67d52a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -998,6 +998,10 @@ bool blk_attempt_plug_merge(struct request_queue *q, 
struct bio *bio,
        if (!plug || rq_list_empty(&plug->mq_list))
                return false;
 
+       rq = plug->mq_list.tail;
+       if (rq->q == q)
+               return blk_attempt_bio_merge(q, rq, bio, nr_segs, false) == 
BIO_MERGE_OK;
+
        rq_list_for_each(&plug->mq_list, rq) {
                if (rq->q == q) {
                        if (blk_attempt_bio_merge(q, rq, bio, nr_segs, false) ==

-- 
Jens Axboe