[PATCH V2 1/3] block: use bvec iterator helper for bio_may_need_split()
From: Ming Lei <hidden>
Date: 2025-12-31 03:01:20
Also in:
io-uring
Subsystem:
block layer, the rest · Maintainers:
Jens Axboe, Linus Torvalds
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/ (local) [1] Signed-off-by: Ming Lei <redacted> --- 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