Re: [PATCH V12 04/20] block: don't use bio->bi_vcnt to figure out segment number

[Omar Sandoval]

On Mon, Nov 26, 2018 at 10:17:04AM +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").
> 
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>

Reviewed-by: Omar Sandoval <osandov@xxxxxx>

> Fixes: 76d8137a3113 ("blk-merge: recaculate segment if it isn't less than max segments")
> 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 e69d8f8ba819..51ec6ca56a0a 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -367,13 +367,7 @@ 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
>