# \[PATCH 01/16\] block: split bio\_iov\_iter\_bounce\_write

- From: Christoph Hellwig \<hch@lst.de\>
- Date: 2026-09-09 06:09:35 
- List: linux-block
- Page: https://ratatoskr.run/linux-block/2026/09/17535463
- Thread: https://ratatoskr.run/linux-block/2026/09/17535462/t.md

```text
Factor out a bio_alloc_bounce_folios helper that we'll use for a
different take on read-side bounce buffering soon.

For that make it and also bio_free_folios available to callers outside of
bio.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 block/bio.c         | 58 ++++++++++++++++++++++++++-------------------
 include/linux/bio.h |  2 ++
 2 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index b89c02f363ce..5792a059ef2a 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1356,7 +1356,7 @@ static struct folio *folio_alloc_greedy(gfp_t gfp, size_t *size,
 	return folio_alloc(gfp, get_order(*size));
 }
 
-static void bio_free_folios(struct bio *bio)
+void bio_free_folios(struct bio *bio)
 {
 	struct bio_vec *bv;
 	int i;
@@ -1369,11 +1369,8 @@ static void bio_free_folios(struct bio *bio)
 	}
 }
 
-static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
-		size_t maxlen, size_t minsize)
+int bio_alloc_bounce_folios(struct bio *bio, size_t total_len, size_t minsize)
 {
-	size_t total_len = min(maxlen, iov_iter_count(iter));
-
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return -EINVAL;
 	if (WARN_ON_ONCE(bio->bi_iter.bi_size))
@@ -1383,7 +1380,6 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
 
 	do {
 		size_t this_len = min(total_len, SZ_1M);
-		size_t copied;
 		struct folio *folio;
 
 		if (this_len > minsize * 2)
@@ -1404,32 +1400,44 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
 		 */
 		this_len &= ~(minsize - 1);
 		bio_add_folio_nofail(bio, folio, this_len, 0);
+		total_len -= this_len;
+	} while (total_len && bio->bi_vcnt < bio->bi_max_vecs);
+
+	if (!bio->bi_iter.bi_size)
+		return -ENOMEM;
+	return 0;
+}
+
+static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
+		size_t maxlen, size_t minsize)
+{
+	size_t total_len = min(maxlen, iov_iter_count(iter));
+	size_t total_copied = 0;
+	struct bio_vec *bv;
+	int i, error;
+
+	error = bio_alloc_bounce_folios(bio, total_len, minsize);
+	if (error)
+		return error;
+
+	bio_for_each_bvec_all(bv, bio, i) {
+		struct folio *folio = page_folio(bv->bv_page);
+		size_t copied;
 
 		if (iter->nofault)
-			copied = copy_folio_from_iter_atomic(folio, 0, this_len,
-							     iter);
+			copied = copy_folio_from_iter_atomic(folio, 0,
+					bv->bv_len, iter);
 		else
-			copied = copy_folio_from_iter(folio, 0, this_len, iter);
-		if (copied < this_len) {
-			/*
-			 * Need to revert the iov iter for all bytes we have
-			 * copied.
-			 *
-			 * However the bio size differs from the real copied
-			 * bytes as @this_len is queued but only advanced
-			 * less than that.
-			 * Need to compensate that for the revert.
-			 */
-			iov_iter_revert(iter, bio->bi_iter.bi_size - this_len +
-					copied);
+			copied = copy_folio_from_iter(folio, 0, bv->bv_len,
+					iter);
+		total_copied += copied;
+		if (copied < bv->bv_len) {
+			iov_iter_revert(iter, total_copied);
 			bio_free_folios(bio);
 			return -EFAULT;
 		}
-		total_len -= this_len;
-	} while (total_len && bio->bi_vcnt < bio->bi_max_vecs);
+	}
 
-	if (!bio->bi_iter.bi_size)
-		return -ENOMEM;
 	return 0;
 }
 
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 0070be355181..584b6abf6baf 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -527,6 +527,8 @@ extern void bio_check_pages_dirty(struct bio *bio);
 int bio_iov_iter_bounce(struct bio *bio, struct iov_iter *iter, size_t maxlen,
 		size_t minsize);
 void bio_iov_iter_unbounce(struct bio *bio, bool is_error, bool mark_dirty);
+int bio_alloc_bounce_folios(struct bio *bio, size_t total_len, size_t minsize);
+void bio_free_folios(struct bio *bio);
 
 extern void bio_copy_data(struct bio *dst, struct bio *src);
 extern void bio_free_pages(struct bio *bio);
-- 
2.53.0

```