Re: [Patch v2] block: return the correct bvec when checking for gaps

[Ming Lei]

On Fri, Jun 04, 2021 at 04:37:19PM -0700, longli@xxxxxxxxxxxxxxxxx wrote:
> From: Long Li <longli@xxxxxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Cc: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> Cc: Pavel Begunkov <asml.silence@xxxxxxxxx>
> Cc: Ming Lei <ming.lei@xxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Cc: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 07173c3ec276 ("block: enable multipage bvecs")
> Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> ---
> Change from v1: add commit details on how data corruption happens

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

-- 
Ming