6.15-stable review patch. If anyone has any objections, please let me know.
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From: Jens Axboe axboe@kernel.dk
commit 961296e89dc3800e6a3abc3f5d5bb4192cf31e98 upstream.
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.co... Reported-by: Hazem Mohamed Abuelfotoh abuehaze@amazon.com Fixes: e70c301faece ("block: don't reorder requests in blk_add_rq_to_plug") Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- block/blk-merge.c | 26 +++++++++++++------------- 1 file changed, 13 insertions(+), 13 deletions(-)
--- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -1127,20 +1127,20 @@ bool blk_attempt_plug_merge(struct reque 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; }