[PATCHv2] block: save page offset gaps in cloned bio

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

The cloned bio needs to inherit the accumulated gaps between vectors so
that we can know if this bio can subscribe to the iova coalescing
optimization.

When cloning for a split, the gap only applies to the front bio since
that's as far as has been processed. The remaining bio can reset its
gaps to 0 since it advanced past the checked vectors, and will start its
accounting from there on the next split check.

Fixes: 2f6b2565d43c ("block: accumulate memory segment gaps per bio")
Reported-by: Eric Auger <eauger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
---
v1->v2:
  Added code comment explaining the reinitialization, and a more clear
  change log.

 block/bio.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/block/bio.c b/block/bio.c
index 898b2f5ef8c81..f95b63c0604af 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -859,6 +859,7 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
 	bio->bi_ioprio = bio_src->bi_ioprio;
 	bio->bi_write_hint = bio_src->bi_write_hint;
 	bio->bi_write_stream = bio_src->bi_write_stream;
+	bio->bi_bvec_gap_bit = bio_src->bi_bvec_gap_bit;
 	bio->bi_iter = bio_src->bi_iter;
 	bio->bi_io_vec = bio_src->bi_io_vec;
 
@@ -1972,6 +1973,14 @@ struct bio *bio_split(struct bio *bio, int sectors,
 
 	bio_advance(bio, split->bi_iter.bi_size);
 
+	/*
+	 * The gap bit is set when splitting to limits and only applies to the
+	 * front bio that was split off. The remaining bio will calcualte its
+	 * gap value when it is subsequently split to limits, so it is safe to
+	 * re-initialize the value back to 0.
+	 */
+	bio->bi_bvec_gap_bit = 0;
+
 	if (bio_flagged(bio, BIO_TRACE_COMPLETION))
 		bio_set_flag(split, BIO_TRACE_COMPLETION);
 
-- 
2.53.0-Meta