linux-kernel - [PATCH V4 12/45] blk-merge: compute bio->bi_seg_front

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Message-Id: <20171218122247.3488-13-ming.lei@redhat.com>
Date:   Mon, 18 Dec 2017 20:22:14 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...com>, Christoph Hellwig <hch@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kent Overstreet <kent.overstreet@...il.com>
Cc:     Huang Ying <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Theodore Ts'o <tytso@....edu>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Coly Li <colyli@...e.de>, Filipe Manana <fdmanana@...il.com>,
        Ming Lei <ming.lei@...hat.com>
Subject: [PATCH V4 12/45] blk-merge: compute bio->bi_seg_front_size efficiently

It is enough to check and compute bio->bi_seg_front_size just
after the 1st segment is found, but current code checks that
for each bvec, which is inefficient.

This patch follows the way in  __blk_recalc_rq_segments()
for computing bio->bi_seg_front_size, and it is more efficient
and code becomes more readable too.

Signed-off-by: Ming Lei <ming.lei@...hat.com>
---
 block/blk-merge.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index f5dedd57dff6..a476337a8ff4 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -146,22 +146,21 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 			bvprvp = &bvprv;
 			sectors += bv.bv_len >> 9;
 
-			if (nsegs == 1 && seg_size > front_seg_size)
-				front_seg_size = seg_size;
 			continue;
 		}
 new_segment:
 		if (nsegs == queue_max_segments(q))
 			goto split;
 
+		if (nsegs == 1 && seg_size > front_seg_size)
+			front_seg_size = seg_size;
+
 		nsegs++;
 		bvprv = bv;
 		bvprvp = &bvprv;
 		seg_size = bv.bv_len;
 		sectors += bv.bv_len >> 9;
 
-		if (nsegs == 1 && seg_size > front_seg_size)
-			front_seg_size = seg_size;
 	}
 
 	do_split = false;
@@ -174,6 +173,8 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 			bio = new;
 	}
 
+	if (nsegs == 1 && seg_size > front_seg_size)
+		front_seg_size = seg_size;
 	bio->bi_seg_front_size = front_seg_size;
 	if (seg_size > bio->bi_seg_back_size)
 		bio->bi_seg_back_size = seg_size;
-- 
2.9.5