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Date:   Mon, 7 Jun 2021 08:09:38 +0800
From:   Ming Lei <ming.lei@redhat.com>
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>,
        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
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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

Reviewed-by: Ming Lei <ming.lei@redhat.com>

-- 
Ming