block: avoid to use bi_vcnt in bio_may_need_split() #441
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January 8, 2026 09:59
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>
bio_iov_bvec_set() creates a cloned bio that borrows a bvec array from an iov_iter. For cloned bios, bi_vcnt is meaningless because iteration is controlled entirely by bi_iter (bi_idx, bi_size, bi_bvec_done), not by bi_vcnt. Remove the incorrect bi_vcnt assignment. Explicitly initialize bi_iter.bi_idx to 0 to ensure iteration starts at the first bvec. While bi_idx is typically already zero from bio initialization, making this explicit improves clarity and correctness. This change also avoids accessing iter->nr_segs, which is an iov_iter implementation detail that block code should not depend on. Signed-off-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Nitesh Shetty <nj.shetty@samsung.com>
io_import_kbuf() recalculates iter->nr_segs to reflect only the bvecs needed for the requested byte range. This was added to provide an accurate segment count to bio_iov_bvec_set(), which copied nr_segs to bio->bi_vcnt for use as a bio split hint. The previous two patches eliminated this dependency: - bio_may_need_split() now uses bi_iter instead of bi_vcnt for split decisions - bio_iov_bvec_set() no longer copies nr_segs to bi_vcnt Since nr_segs is no longer used for bio split decisions, the recalculation loop is unnecessary. The iov_iter already has the correct bi_size to cap iteration, so an oversized nr_segs is harmless. Link: https://lkml.org/lkml/2025/4/16/351 Signed-off-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Nitesh Shetty <nj.shetty@samsung.com>
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Pull request for series with
subject: block: avoid to use bi_vcnt in bio_may_need_split()
version: 2
url: https://patchwork.kernel.org/project/linux-block/list/?series=1037526