linux-kernel - Re: [Patch v3] block: return the correct bvec when checking for gaps

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Message-ID: <6691cb0d-d368-7115-87f2-ae5950699b7d@kernel.dk>
Date:   Tue, 8 Jun 2021 15:07:15 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     longli@...uxonhyperv.com, linux-block@...r.kernel.org
Cc:     Long Li <longli@...rosoft.com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Pavel Begunkov <asml.silence@...il.com>,
        Ming Lei <ming.lei@...hat.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 v3] block: return the correct bvec when checking for gaps

On 6/7/21 1:34 PM, 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. 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.

Applied, thanks.

-- 
Jens Axboe