# \[PATCH 10/16\] iomap: better read bounce buffering support

- From: Christoph Hellwig \<hch@lst.de\>
- Date: 2026-09-09 06:10:35 
- List: linux-block
- Page: https://ratatoskr.run/linux-block/2026/09/17535472
- Thread: https://ratatoskr.run/linux-block/2026/09/17535462/t.md

```text
Add helpers to bounce buffer an upper bio into one or more lower bios
using bounce buffers, and to copy the data back on completion.

Compared to the existing IOMAP_DIO_BOUNCE support for read bios, this
has two advantages:  by removing the special bounce bio_vec it allows
to the full and "round" size of a single bio, i.e., 1MiB when using
4k pages.  This is important for good performance on HDD.  Additionally
it allows to bounce buffer a bio from completion conext, and thus
implement a "lazy" bounce buffering scheme, where the data is only
read into a bounce buffer after an initial checksum validation failure,
thus avoiding the bounce buffering I/O for most I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/iomap/ioend.c      | 90 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/iomap.h |  5 +++
 2 files changed, 95 insertions(+)

diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
index 332dbfb2230f..ba97eec4ee2c 100644
--- a/fs/iomap/ioend.c
+++ b/fs/iomap/ioend.c
@@ -514,6 +514,96 @@ struct iomap_ioend *iomap_split_ioend(struct iomap_ioend *ioend,
 }
 EXPORT_SYMBOL_GPL(iomap_split_ioend);
 
+void iomap_bounce_read(struct iomap_ioend *orig_ioend, unsigned int minsize,
+		void (*submit_ioend)(struct iomap_ioend *ioend))
+{
+	struct inode *inode = orig_ioend->io_inode;
+	struct bio *orig_bio = &orig_ioend->io_bio;
+	loff_t file_offset = orig_ioend->io_offset;
+	sector_t sector = orig_ioend->io_sector;
+	size_t total_len = round_up(orig_ioend->io_size, minsize);
+
+	WARN_ON_ONCE(!(orig_ioend->io_flags & IOMAP_IOEND_DIRECT));
+
+	/* We can't poll a bio that is not passed on to hardware */
+	orig_bio->bi_opf &= ~REQ_POLLED;
+
+	do {
+		struct iomap_ioend *ioend;
+		struct bio *bio;
+		int error;
+
+		bio = bio_alloc_bioset(orig_bio->bi_bdev,
+				min(total_len / minsize, BIO_MAX_VECS),
+				orig_bio->bi_opf, GFP_KERNEL,
+				&iomap_ioend_split_bioset);
+		error = bio_alloc_bounce_folios(bio, total_len, minsize);
+		if (error) {
+			bio_put(bio);
+			orig_bio->bi_status = errno_to_blk_status(error);
+			break;
+		}
+		bio->bi_ioprio = orig_bio->bi_ioprio;
+		bio->bi_write_hint = orig_bio->bi_write_hint;
+		bio->bi_write_stream = orig_bio->bi_write_stream;
+		bio->bi_iter.bi_sector = sector;
+
+		ioend = iomap_init_ioend(inode, bio, file_offset,
+				orig_ioend->io_flags);
+
+		total_len -= bio->bi_iter.bi_size;
+		file_offset += bio->bi_iter.bi_size;
+		sector += (bio->bi_iter.bi_size >> SECTOR_SHIFT);
+
+		bio->bi_private = orig_bio;
+		bio_inc_remaining(orig_bio);
+		submit_ioend(ioend);
+	} while (total_len > 0);
+
+	bio_endio(&orig_ioend->io_bio);
+}
+EXPORT_SYMBOL_GPL(iomap_bounce_read);
+
+static void iomap_ioend_unbounce(struct iomap_ioend *orig_ioend,
+		struct iomap_ioend *ioend)
+{
+	struct bio *orig_bio = &orig_ioend->io_bio;
+	struct iov_iter to;
+	struct bio_vec *bv;
+	int i;
+
+	iov_iter_bvec(&to, ITER_DEST, orig_bio->bi_io_vec, orig_bio->bi_vcnt,
+			orig_ioend->io_size);
+	to.iov_offset = orig_ioend->io_bvec_offset;
+
+	if (ioend->io_offset != orig_ioend->io_offset) {
+		WARN_ON_ONCE(ioend->io_offset < orig_ioend->io_offset);
+		iov_iter_advance(&to, ioend->io_offset - orig_ioend->io_offset);
+	}
+
+	/* copying to pinned pages should always work */
+	bio_for_each_bvec_all(bv, &ioend->io_bio, i)
+		WARN_ON_ONCE(copy_to_iter(bvec_virt(bv), bv->bv_len, &to) !=
+				bv->bv_len);
+}
+
+void iomap_bounce_read_end_io(struct iomap_ioend *ioend, struct bio *orig_bio,
+		int error)
+{
+	if (error)
+		orig_bio->bi_status = errno_to_blk_status(error);
+	else
+		iomap_ioend_unbounce(iomap_ioend_from_bio(orig_bio), ioend);
+
+	bio_free_folios(&ioend->io_bio);
+	if (bio_integrity(&ioend->io_bio))
+		fs_bio_integrity_free(&ioend->io_bio);
+	bio_put(&ioend->io_bio);
+
+	bio_endio(orig_bio);
+}
+EXPORT_SYMBOL_GPL(iomap_bounce_read_end_io);
+
 static int __init iomap_ioend_init(void)
 {
 	const unsigned int nr_mempool_entries = 4 * (PAGE_SIZE / SECTOR_SIZE);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index bffdc217ad85..fbe051f0b032 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -607,6 +607,11 @@ void iomap_finish_folio_write(struct inode *inode, struct folio *folio,
 int iomap_writeback_folio(struct iomap_writepage_ctx *wpc, struct folio *folio);
 int iomap_writepages(struct iomap_writepage_ctx *wpc);
 
+void iomap_bounce_read(struct iomap_ioend *orig_ioend, unsigned int minsize,
+		void (*submit_ioend)(struct iomap_ioend *ioend));
+void iomap_bounce_read_end_io(struct iomap_ioend *ioend, struct bio *orig_bio,
+		int error);
+
 struct iomap_read_folio_ctx {
 	const struct iomap_read_ops *ops;
 	struct folio		*cur_folio;
-- 
2.53.0

```