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

Tim Gardner tim.gardner at canonical.com
Tue Jul 6 17:14:56 UTC 2021

From: Long Li <longli at microsoft.com>

BugLink: https://bugs.launchpad.net/bugs/1931497

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 at kernel.dk>
Cc: Johannes Thumshirn <johannes.thumshirn at wdc.com>
Cc: Pavel Begunkov <asml.silence at gmail.com>
Cc: Ming Lei <ming.lei at redhat.com>
Cc: Tejun Heo <tj at kernel.org>
Cc: "Matthew Wilcox (Oracle)" <willy at infradead.org>
Cc: Jeffle Xu <jefflexu at linux.alibaba.com>
Cc: linux-kernel at vger.kernel.org
Cc: stable at vger.kernel.org
Fixes: 07173c3ec276 ("block: enable multipage bvecs")
Signed-off-by: Long Li <longli at microsoft.com>
Reviewed-by: Ming Lei <ming.lei at redhat.com>
Reviewed-by: Christoph Hellwig <hch at lst.de>
Link: https://lore.kernel.org/r/1623094445-22332-1-git-send-email-longli@linuxonhyperv.com
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry picked from commit c9c9762d4d44dcb1b2ba90cfb4122dc11ceebf31)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
 include/linux/bio.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 853d92ceee64e..b0927b27a8a88 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -40,9 +40,6 @@
 #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)))
@@ -246,7 +243,7 @@ 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)
@@ -254,10 +251,9 @@ 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);
-		return;
-	}
+	bio_get_first_bvec(bio, bv);
+	if (bv->bv_len == bio->bi_iter.bi_size)
+		return;		/* this bio only has a single bvec */
 	bio_advance_iter(bio, &iter, iter.bi_size);

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