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Date:   Mon, 7 Jun 2021 08:09:00 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     longli@linuxonhyperv.com
Cc:     linux-block@vger.kernel.org, Long Li <longli@microsoft.com>,
        Jens Axboe <axboe@kernel.dk>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Ming Lei <ming.lei@redhat.com>, Tejun Heo <tj@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Jeffle Xu <jefflexu@linux.alibaba.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [Patch v2] block: return the correct bvec when checking for gaps
Message-ID: <YL3GDF5KiM9e69eW@infradead.org>
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On Fri, Jun 04, 2021 at 04:37:19PM -0700, longli@linuxonhyperv.com wrote:
> From: Long Li <longli@microsoft.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. If the pages in the bvec go across the
> seg_boundary_mask, this check for merging can potentially succeed if only
> the 1st page is tested, and can fail if all the pages are tested.
> 
> Later, when SCSI builds the SG list the same check for merging is done in
> __blk_segment_map_sg_merge() with all the pages in the bvec tested. This
> time the check may fail if the pages in bvec go across the
> seg_boundary_mask (but tested okay in bio_will_gap() earlier, so those
> BIOs were merged). If this check fails, we end up with a broken SG list
> for drivers assuming the SG list not having offsets in intermediate pages.
> This results in incorrect pages written to the disk.
> 
> Fix this by returning the multi-page bvec when testing gaps for merging.
> 
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Cc: Pavel Begunkov <asml.silence@gmail.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Fixes: 07173c3ec276 ("block: enable multipage bvecs")
> Signed-off-by: Long Li <longli@microsoft.com>
> ---
> Change from v1: add commit details on how data corruption happens
> 
>  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;

Nit: I'd move the comment a bit as the current placement confused me at
first.  Also maybe use bio_get_first_bvec here to make it even more
obvious:

	bio_get_first_bvec(bio, bv);
	if (bv->bv_len == bio->bi_iter.bi_size)
		return;		/* this bio only has a single bvec */