[PATCH 3/3] block: improve aligning down bios in bio_iov_iter_bounce_write
From: Christoph Hellwig <hch@lst.de>
Date: 2026-09-07 07:41:37
Subsystem:
block layer, the rest · Maintainers:
Jens Axboe, Linus Torvalds
Sashiko complained about bio_iov_iter_align_down potentially dropping
bvecs, and thus losing bounce folios. While losing bio_vecs is real,
bio_iov_iter_bounce_write allocates all segments and thus the entire bio
in multiples of the minsize, thus making the rounding down redundant.
Replace it with a safety rounding down of this_len in case something odd
happens to the greedy folio allocation helper.
Fixes: e7b8b3c5b2a6 ("block: align down bounces bios")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
block/bio.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/block/bio.c b/block/bio.c
index f95b63c0604a..57ee335899f3 100644
--- a/block/bio.c
+++ b/block/bio.c@@ -1389,6 +1389,14 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter, folio = folio_alloc_greedy(GFP_KERNEL, &this_len, minsize); if (!folio) break; + + /* + * Align down the size to the minimum alignment. In practice + * this should not happen as minsize is expected to be a power + * of two, as is the allocation size, but it offers us a cheap + * extra safety belt. + */ + this_len &= ~(minsize - 1); bio_add_folio_nofail(bio, folio, this_len, 0); if (iter->nofault)
@@ -1416,8 +1424,7 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter, if (!bio->bi_iter.bi_size) return -ENOMEM; - return bio_iov_iter_align_down(bio, iter, - &bio->bi_io_vec[bio->bi_vcnt - 1], minsize - 1); + return 0; } static int bio_iov_iter_bounce_read(struct bio *bio, struct iov_iter *iter,
--
2.53.0