Sorry, Jens, I think I didn't get the point. Do I miss anything ? On 02/01/2018 11:07 AM, Jens Axboe wrote: > Yeah I agree, and my last patch missed that we do care about segments for > discards. Below should be better... > > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 8452fc7164cc..055057bd727f 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -553,9 +553,8 @@ static bool req_no_special_merge(struct request *req) > static int ll_merge_requests_fn(struct request_queue *q, struct request *req, > struct request *next) > { > - int total_phys_segments; > - unsigned int seg_size = > - req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size; > + int total_phys_segments = req->nr_phys_segments + > + next->nr_phys_segments; For DISCARD reqs, the total_phys_segments is still zero here. > > /* > * First check if the either of the requests are re-queued > @@ -574,8 +573,15 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req, > blk_rq_get_max_sectors(req, blk_rq_pos(req))) > return 0; > > - total_phys_segments = req->nr_phys_segments + next->nr_phys_segments; > - if (blk_phys_contig_segment(q, req->biotail, next->bio)) { > + /* > + * If the requests aren't carrying any data payloads, we don't need > + * to look at the segment count > + */ > + if (bio_has_data(next->bio) && > + blk_phys_contig_segment(q, req->biotail, next->bio)) { > + unsigned int seg_size = req->biotail->bi_seg_back_size + > + next->bio->bi_seg_front_size; Yes, total_phys_segments will not be decreased. > + > if (req->nr_phys_segments == 1) > req->bio->bi_seg_front_size = seg_size; > if (next->nr_phys_segments == 1) > @@ -584,7 +590,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req, > } > > if (total_phys_segments > queue_max_segments(q)) > - return 0; > + return 0; > > if (blk_integrity_merge_rq(q, req, next) == false) > return 0; But finally, the merged DISCARD req's nr_phys_segment is still zero. blk_rq_nr_discard_segments will return 1 but this req has two bios. blk_rq_nr_discard_segments 's comment says -- Each discard bio merged into a request is counted as one segment -- Maybe patch below should be followed with yours. diff --git a/block/blk-core.c b/block/blk-core.c index a2005a4..b444fb7 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -1763,7 +1763,6 @@ bool bio_attempt_discard_merge(struct request_queue *q, struct request *req, req->biotail = bio; req->__data_len += bio->bi_iter.bi_size; req->ioprio = ioprio_best(req->ioprio, bio_prio(bio)); - req->nr_phys_segments = segments + 1; blk_account_io_start(req, false); return true; diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 4f3df80..1af2138 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -1312,7 +1312,13 @@ static inline unsigned short blk_rq_nr_phys_segments(struct request *rq) */ static inline unsigned short blk_rq_nr_discard_segments(struct request *rq) { - return max_t(unsigned short, rq->nr_phys_segments, 1); + struct bio *bio; + unsigned short count = 0; + + __rq_for_each_bio(bio, req) + count ++; + + return count; } Thanks Jianchao