linux-ext4 - Re: [PATCH V12 04/20] block: don't use bio->bi

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Date:   Mon, 26 Nov 2018 14:14:47 -0800
From:   Omar Sandoval <osandov@...ndov.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Theodore Ts'o <tytso@....edu>, Omar Sandoval <osandov@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, David Sterba <dsterba@...e.com>,
        linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        Christoph Hellwig <hch@....de>, linux-ext4@...r.kernel.org,
        Coly Li <colyli@...e.de>, linux-bcache@...r.kernel.org,
        Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V12 04/20] block: don't use bio->bi_vcnt to figure out
 segment number

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@....de>

Reviewed-by: Omar Sandoval <osandov@...com>

> Fixes: 76d8137a3113 ("blk-merge: recaculate segment if it isn't less than max segments")
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  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
>