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

[Nitesh Shetty <nj.shetty@samsung.com>]

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On 31/12/25 11:00AM, Ming Lei wrote:
>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>

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