On Thu, Nov 15, 2018 at 04:53:04PM +0800, Ming Lei wrote:
> It is wrong to use bio->bi_vcnt to figure out how many segments
> there are in the bio even though CLONED flag isn't set on this bio,
> because this bio may be splitted or advanced.
>
> So always use bio_segments() in blk_recount_segments(), and it shouldn't
> cause any performance loss now because the physical segment number is figured
> out in blk_queue_split() and BIO_SEG_VALID is set meantime since
> bdced438acd83ad83a6c ("block: setup bi_phys_segments after splitting").
>
> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> Fixes: 7f60dcaaf91 ("block: blk-merge: fix blk_recount_segments()")
>From what I can tell, the problem was originally introduced by
76d8137a3113 ("blk-merge: recaculate segment if it isn't less than max segments")
Is that right?
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
> Cc: dm-devel@xxxxxxxxxx
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: linux-raid@xxxxxxxxxxxxxxx
> Cc: linux-erofs@xxxxxxxxxxxxxxxx
> Cc: David Sterba <dsterba@xxxxxxxx>
> Cc: linux-btrfs@xxxxxxxxxxxxxxx
> Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Cc: linux-xfs@xxxxxxxxxxxxxxx
> Cc: Gao Xiang <gaoxiang25@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Cc: linux-ext4@xxxxxxxxxxxxxxx
> Cc: Coly Li <colyli@xxxxxxx>
> Cc: linux-bcache@xxxxxxxxxxxxxxx
> Cc: Boaz Harrosh <ooo@xxxxxxxxxxxxxxx>
> Cc: Bob Peterson <rpeterso@xxxxxxxxxx>
> Cc: cluster-devel@xxxxxxxxxx
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> block/blk-merge.c | 8 +-------
> 1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index cb9f49bcfd36..153a659fde74 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -429,13 +429,7 @@ void blk_recalc_rq_segments(struct request *rq)
>
> void blk_recount_segments(struct request_queue *q, struct bio *bio)
> {
> - unsigned short seg_cnt;
> -
> - /* estimate segment number by bi_vcnt for non-cloned bio */
> - if (bio_flagged(bio, BIO_CLONED))
> - seg_cnt = bio_segments(bio);
> - else
> - seg_cnt = bio->bi_vcnt;
> + unsigned short seg_cnt = bio_segments(bio);
>
> if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags) &&
> (seg_cnt < queue_max_segments(q)))
> --
> 2.9.5
>
