linux-kernel - Re: [PATCH] block: return the correct bvec when checking for gaps

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Date:   Fri, 4 Jun 2021 09:53:15 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     longli@...uxonhyperv.com
Cc:     linux-block@...r.kernel.org, Long Li <longli@...rosoft.com>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Pavel Begunkov <asml.silence@...il.com>,
        Tejun Heo <tj@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Jeffle Xu <jefflexu@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] block: return the correct bvec when checking for gaps

Hello Long,

On Thu, Jun 03, 2021 at 03:34:31PM -0700, longli@...uxonhyperv.com wrote:
> From: Long Li <longli@...rosoft.com>
> 
> After commit 07173c3ec276 ("block: enable multipage bvecs"), a bvec can
> have multiple pages. But bio_will_gap() still assumes one page bvec while
> checking for merging. This causes data corruption on drivers relying on
> the correct merging on virt_boundary_mask.

Can you explain the data corruption a bit? 

IMO, either single page bvec or multipage bvec should be fine, because
bio_will_gap() just checks if the last bvec of prev bio and the 1st bvec
of next bio can be merged.

> 
> Fix this by returning the multi-page bvec for testing gaps for merging.
> 
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Johannes Thumshirn <johannes.thumshirn@....com>
> Cc: Pavel Begunkov <asml.silence@...il.com>
> Cc: Ming Lei <ming.lei@...hat.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Cc: Jeffle Xu <jefflexu@...ux.alibaba.com>
> Cc: linux-kernel@...r.kernel.org
> Cc: stable@...r.kernel.org
> Fixes: 07173c3ec276 ("block: enable multipage bvecs")
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  include/linux/bio.h | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index a0b4cfdf62a4..6b2f609ccfbf 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -44,9 +44,6 @@ static inline unsigned int bio_max_segs(unsigned int nr_segs)
>  #define bio_offset(bio)		bio_iter_offset((bio), (bio)->bi_iter)
>  #define bio_iovec(bio)		bio_iter_iovec((bio), (bio)->bi_iter)
>  
> -#define bio_multiple_segments(bio)				\
> -	((bio)->bi_iter.bi_size != bio_iovec(bio).bv_len)
> -
>  #define bvec_iter_sectors(iter)	((iter).bi_size >> 9)
>  #define bvec_iter_end_sector(iter) ((iter).bi_sector + bvec_iter_sectors((iter)))
>  
> @@ -271,7 +268,7 @@ static inline void bio_clear_flag(struct bio *bio, unsigned int bit)
>  
>  static inline void bio_get_first_bvec(struct bio *bio, struct bio_vec *bv)
>  {
> -	*bv = bio_iovec(bio);
> +	*bv = mp_bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
>  }
>  
>  static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
> @@ -279,10 +276,10 @@ static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
>  	struct bvec_iter iter = bio->bi_iter;
>  	int idx;
>  
> -	if (unlikely(!bio_multiple_segments(bio))) {
> -		*bv = bio_iovec(bio);
> +	/* this bio has only one bvec */
> +	*bv = mp_bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
> +	if (bv->bv_len == bio->bi_iter.bi_size)
>  		return;
> -	}
>  
>  	bio_advance_iter(bio, &iter, iter.bi_size);

The patch itself looks fine, given both bio_get_first_bvec() and bio_get_last_bvec()
are used in bio_will_gap() only.


Thanks,
Ming