block: kill QUEUE_FLAG_NO_SG_MERGE

Since bdced438acd83ad83a6c ("block: setup bi_phys_segments after splitting"),
physical segment number is mainly figured out in blk_queue_split() for
fast path, and the flag of BIO_SEG_VALID is set there too.

Now only blk_recount_segments() and blk_recalc_rq_segments() use this
flag.

Basically blk_recount_segments() is bypassed in fast path given BIO_SEG_VALID
is set in blk_queue_split().

For another user of blk_recalc_rq_segments():

- run in partial completion branch of blk_update_request, which is an unusual case

- run in blk_cloned_rq_check_limits(), still not a big problem if the flag is killed
since dm-rq is the only user.

Multi-page bvec is enabled now, not doing S/G merging is rather pointless with the
current setup of the I/O path, as it isn't going to save you a significant amount
of cycles.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 6 insertions, 25 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 1912499b08b7..bed065904677 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -358,8 +358,7 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
 EXPORT_SYMBOL(blk_queue_split);
 
 static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
-                                             struct bio *bio,
-                                             bool no_sg_merge)
+                                             struct bio *bio)
 {
         struct bio_vec bv, bvprv = { NULL };
         int prev = 0;
@@ -385,13 +384,6 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
         nr_phys_segs = 0;
         for_each_bio(bio) {
                 bio_for_each_bvec(bv, bio, iter) {
-                        /*
-                         * If SG merging is disabled, each bio vector is
-                         * a segment
-                         */
-                        if (no_sg_merge)
-                                goto new_segment;
-
                         if (prev) {
                                 if (seg_size + bv.bv_len
                                     > queue_max_segment_size(q))
@@ -421,27 +413,16 @@ new_segment:
 
 void blk_recalc_rq_segments(struct request *rq)
 {
-        bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE,
-                        &rq->q->queue_flags);
-
-        rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio,
-                        no_sg_merge);
+        rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio);
 }
 
 void blk_recount_segments(struct request_queue *q, struct bio *bio)
 {
-        unsigned short seg_cnt = bio_segments(bio);
-
-        if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
-                        (seg_cnt < queue_max_segments(q)))
-                bio->bi_phys_segments = seg_cnt;
-        else {
-                struct bio *nxt = bio->bi_next;
+        struct bio *nxt = bio->bi_next;
 
-                bio->bi_next = NULL;
-                bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, false);
-                bio->bi_next = nxt;
-        }
+        bio->bi_next = NULL;
+        bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio);
+        bio->bi_next = nxt;
 
         bio_set_flag(bio, BIO_SEG_VALID);
 }