block: setup bi_phys_segments after splitting

The number of bio->bi_phys_segments is always obtained
during bio splitting, so it is natural to setup it
just after bio splitting, then we can avoid to compute
nr_segment again during merge.
Reviewed-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-merge.c

@@ -11,13 +11,16 @@
 
 static struct bio *blk_bio_discard_split(struct request_queue *q,
 					 struct bio *bio,
-					 struct bio_set *bs)
+					 struct bio_set *bs,
+					 unsigned *nsegs)
 {
 	unsigned int max_discard_sectors, granularity;
 	int alignment;
 	sector_t tmp;
 	unsigned split_sectors;
 
+	*nsegs = 1;
+
 	/* Zero-sector (unknown) and one-sector granularities are the same.  */
 	granularity = max(q->limits.discard_granularity >> 9, 1U);
 
@@ -51,8 +54,11 @@ static struct bio *blk_bio_discard_split(struct request_queue *q,
 
 static struct bio *blk_bio_write_same_split(struct request_queue *q,
 					    struct bio *bio,
-					    struct bio_set *bs)
+					    struct bio_set *bs,
+					    unsigned *nsegs)
 {
+	*nsegs = 1;
+
 	if (!q->limits.max_write_same_sectors)
 		return NULL;
 
@@ -64,7 +70,8 @@ static struct bio *blk_bio_write_same_split(struct request_queue *q,
 
 static struct bio *blk_bio_segment_split(struct request_queue *q,
 					 struct bio *bio,
-					 struct bio_set *bs)
+					 struct bio_set *bs,
+					 unsigned *segs)
 {
 	struct bio_vec bv, bvprv, *bvprvp = NULL;
 	struct bvec_iter iter;
@@ -106,22 +113,30 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 		sectors += bv.bv_len >> 9;
 	}
 
+	*segs = nsegs;
 	return NULL;
 split:
+	*segs = nsegs;
 	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
 void blk_queue_split(struct request_queue *q, struct bio **bio,
 		     struct bio_set *bs)
 {
-	struct bio *split;
+	struct bio *split, *res;
+	unsigned nsegs;
 
 	if ((*bio)->bi_rw & REQ_DISCARD)
-		split = blk_bio_discard_split(q, *bio, bs);
+		split = blk_bio_discard_split(q, *bio, bs, &nsegs);
 	else if ((*bio)->bi_rw & REQ_WRITE_SAME)
-		split = blk_bio_write_same_split(q, *bio, bs);
+		split = blk_bio_write_same_split(q, *bio, bs, &nsegs);
 	else
-		split = blk_bio_segment_split(q, *bio, q->bio_split);
+		split = blk_bio_segment_split(q, *bio, q->bio_split, &nsegs);
+
+	/* physical segments can be figured out during splitting */
+	res = split ? split : *bio;
+	res->bi_phys_segments = nsegs;
+	bio_set_flag(res, BIO_SEG_VALID);
 
 	if (split) {
 		bio_chain(split, *bio);