[PATCH V2 1/3] block: use bvec iterator helper for bio_may_need_split()

[Ming Lei <ming.lei@redhat.com>]

bio_may_need_split() uses bi_vcnt to determine if a bio has a single
segment, but bi_vcnt is unreliable for cloned bios. Cloned bios share
the parent's bi_io_vec array but iterate over a subset via bi_iter,
so bi_vcnt may not reflect the actual segment count being iterated.

Replace the bi_vcnt check with bvec iterator access via
__bvec_iter_bvec(), comparing bi_iter.bi_size against the current
bvec's length. This correctly handles both cloned and non-cloned bios.

Move bi_io_vec into the first cache line adjacent to bi_iter. This is
a sensible layout since bi_io_vec and bi_iter are commonly accessed
together throughout the block layer - every bvec iteration requires
both fields. This displaces bi_end_io to the second cache line, which
is acceptable since bi_end_io and bi_private are always fetched
together in bio_endio() anyway.

The struct layout change requires bio_reset() to preserve and restore
bi_io_vec across the memset, since it now falls within BIO_RESET_BYTES.

Nitesh verified that this patch doesn't regress NVMe 512-byte IO perf [1].

Link: https://lore.kernel.org/linux-block/20251220081607.tvnrltcngl3cc2fh@green245.gost/ [1]
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/bio.c               |  3 +++
 block/blk.h               | 12 +++++++++---
 include/linux/blk_types.h |  4 ++--
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index e726c0e280a8..0e936288034e 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -301,9 +301,12 @@ EXPORT_SYMBOL(bio_init);
  */
 void bio_reset(struct bio *bio, struct block_device *bdev, blk_opf_t opf)
 {
+	struct bio_vec          *bv = bio->bi_io_vec;
+
 	bio_uninit(bio);
 	memset(bio, 0, BIO_RESET_BYTES);
 	atomic_set(&bio->__bi_remaining, 1);
+	bio->bi_io_vec = bv;
 	bio->bi_bdev = bdev;
 	if (bio->bi_bdev)
 		bio_associate_blkg(bio);
diff --git a/block/blk.h b/block/blk.h
index e4c433f62dfc..98f4dfd4ec75 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -371,12 +371,18 @@ struct bio *bio_split_zone_append(struct bio *bio,
 static inline bool bio_may_need_split(struct bio *bio,
 		const struct queue_limits *lim)
 {
+	const struct bio_vec *bv;
+
 	if (lim->chunk_sectors)
 		return true;
-	if (bio->bi_vcnt != 1)
+
+	if (!bio->bi_io_vec)
+		return true;
+
+	bv = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
+	if (bio->bi_iter.bi_size > bv->bv_len)
 		return true;
-	return bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset >
-		lim->max_fast_segment_size;
+	return bv->bv_len + bv->bv_offset > lim->max_fast_segment_size;
 }
 
 /**
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 5dc061d318a4..19a888a2f104 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -232,6 +232,8 @@ struct bio {
 
 	atomic_t		__bi_remaining;
 
+	/* The actual vec list, preserved by bio_reset() */
+	struct bio_vec		*bi_io_vec;
 	struct bvec_iter	bi_iter;
 
 	union {
@@ -275,8 +277,6 @@ struct bio {
 
 	atomic_t		__bi_cnt;	/* pin count */
 
-	struct bio_vec		*bi_io_vec;	/* the actual vec list */
-
 	struct bio_set		*bi_pool;
 };
 
-- 
2.47.0