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

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

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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 */

Thanks! I'll send v3 with those changes.