RE: [PATCH] block: return the correct bvec when checking for gaps

[Long Li]

> Subject: Re: [PATCH] block: return the correct bvec when checking for gaps
> 
> Hello Long,
> 
> On Thu, Jun 03, 2021 at 03:34:31PM -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. This causes data corruption on
> > drivers relying on the correct merging on virt_boundary_mask.
> 
> Can you explain the data corruption a bit?
> 
> IMO, either single page bvec or multipage bvec should be fine, because
> bio_will_gap() just checks if the last bvec of prev bio and the 1st bvec of next
> bio can be merged.

Hi Ming,

When bio_will_gap() calls into biovec_phys_mergeable (), seg_boundary_mask (queue_segment_boundary()) is used to test if the two bio_vecs can be merged. This test can succeed if only the 1st page in bvec is used, but at the same time it can fail if all the pages in bvec are used. In other words, if the pages in bvec go across the seg_boundary_mask, the test can potentially succeed if only the 1st page is tested, but can fail if all the pages are tested.

Later, when SCSI builds the SG list from BIOs (that calls into __blk_bios_map_sg), __blk_segment_map_sg_merge() calls biovec_phys_mergeable() doing the same test . This time it may fail if the pages in bvec go across the seg_boundary_mask (but tested okay in bio_will_gap() earlier, so those two BIOs were merged). If __blk_segment_map_sg_merge() fails, we end up with a broken SG list for drivers assuming the SG list not having offsets in intermediate pages.

In practice, usually a duplicate page (because merging fails) is put to the SG list. This page and all the pages afterwards in the SG list end up writing to the wrong sectors on disk.

Thanks,
Long

> 
> >
> > Fix this by returning the multi-page bvec for 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>
> > ---
> >  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;
> > -	}
> >
> >  	bio_advance_iter(bio, &iter, iter.bi_size);
> 
> The patch itself looks fine, given both bio_get_first_bvec() and
> bio_get_last_bvec() are used in bio_will_gap() only.
> 
> 
> Thanks,
> Ming