[PATCH 12/22] iomap: better read bounce buffering support

[Christoph Hellwig <hch@lst.de>]

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>
---
 fs/iomap/ioend.c      | 87 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/iomap.h |  5 +++
 2 files changed, 92 insertions(+)

diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
index d43e6229900c..34f71c8b5ebd 100644
--- a/fs/iomap/ioend.c
+++ b/fs/iomap/ioend.c
@@ -515,6 +515,93 @@ 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));
+
+	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 e7bee96379df..ff9afdd39682 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -509,6 +509,11 @@ 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