[PATCH] block: blk-merge: fast-clone bio when splitting rw bios

[Ming Lei]

biovecs has become immutable since v3.13, so it isn't necessary
to allocate biovecs for the new cloned bios, then we can save
one extra biovecs allocation/copy, and the allocation is often
not fixed-length and a bit more expensive.

For example, if the 'max_sectors_kb' of null blk's queue is set
as 16(32 sectors) via sysfs just for making more splits, this patch
can increase throught about ~70% in the sequential read test over
null_blk(direct io, bs: 1M).

Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Ming Lin <ming.l@xxxxxxxxxxxxxxx>
Cc: Dongsu Park <dpark@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxxxxx>
---
 block/blk-merge.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d088cff..be2493f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -66,16 +66,13 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 					 struct bio *bio,
 					 struct bio_set *bs)
 {
-	struct bio *split;
 	struct bio_vec bv, bvprv;
 	struct bvec_iter iter;
 	unsigned seg_size = 0, nsegs = 0, sectors = 0;
 	int prev = 0;
 
 	bio_for_each_segment(bv, bio, iter) {
-		sectors += bv.bv_len >> 9;
-
-		if (sectors > queue_max_sectors(q))
+		if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q))
 			goto split;
 
 		/*
@@ -96,6 +93,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 			seg_size += bv.bv_len;
 			bvprv = bv;
 			prev = 1;
+			sectors += bv.bv_len >> 9;
 			continue;
 		}
 new_segment:
@@ -106,21 +104,12 @@ new_segment:
 		bvprv = bv;
 		prev = 1;
 		seg_size = bv.bv_len;
+		sectors += bv.bv_len >> 9;
 	}
 
 	return NULL;
 split:
-	split = bio_clone_bioset(bio, GFP_NOIO, bs);
-
-	split->bi_iter.bi_size -= iter.bi_size;
-	bio->bi_iter = iter;
-
-	if (bio_integrity(bio)) {
-		bio_integrity_advance(bio, split->bi_iter.bi_size);
-		bio_integrity_trim(split, 0, bio_sectors(split));
-	}
-
-	return split;
+	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
 void blk_queue_split(struct request_queue *q, struct bio **bio,
-- 
1.9.1

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