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/
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>

diff --git a/block/bio.c b/block/bio.c
index e726c0e280a8dc85ef823511f1c59547ada5b3bd..0e936288034e3db13272501a5aed48a2746199b4 100644 (file)
--- 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 e4c433f62dfc7acb94d6a1b3e6234ce6e1f24837..98f4dfd4ec7581ac7fb3f7002f696b30ad585f8e 100644 (file)
--- 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 5dc061d318a45dac1c9f7d8bc0c03acd039bc698..19a888a2f104e8399d872f056f83c3c4f8030fe1 100644 (file)
--- 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;
 };