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block: use bvec iterator helper for bio_may_need_split() 

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>
Reviewed-by: Nitesh Shetty <nj.shetty@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
master
Ming Lei 2025-12-31 11:00:55 +08:00 committed by Jens Axboe
parent 69d26698e4
commit ee623c892a
3 changed files with 14 additions and 5 deletions
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3
block/bio.c
View File

@ -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);

12
block/blk.h
View File

@ -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;
return bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset >
lim->max_fast_segment_size;
bv = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
if (bio->bi_iter.bi_size > bv->bv_len)
return true;
return bv->bv_len + bv->bv_offset > lim->max_fast_segment_size;
}
/**

4
include/linux/blk_types.h
View File

@ -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;
};