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
>
> 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 */
