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    Subject[PATCH v3 21/49] blk-merge: compute bio->bi_seg_front_size efficiently
    Date
    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@redhat.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 99038830fb42..d91f07813dee 100644
    --- a/block/blk-merge.c
    +++ b/block/blk-merge.c
    @@ -145,22 +145,21 @@ 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;
    @@ -173,6 +172,8 @@ 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;
    --
    2.9.4
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