[PATCH 3/3] block: improve aligning down bios in bio_iov_iter_bounce_write

[Christoph Hellwig <hch@lst.de>]

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