block: add helpers to bounce buffer an iov_iter into bios

Add helpers to implement bounce buffering of data into a bio to implement
direct I/O for cases where direct user access is not possible because
stable in-flight data is required.  These are intended to be used as
easily as bio_iov_iter_get_pages for the zero-copy path.

The write side is trivial and just copies data into the bounce buffer.
The read side is a lot more complex because it needs to perform the copy
from the completion context, and without preserving the iov_iter through
the call chain.  It steals a trick from the integrity data user interface
and uses the first vector in the bio for the bounce buffer data that is
fed to the block I/O stack, and uses the others to record the user
buffer fragments.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Tested-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index d32aee2857a9..69d56b1d1bd2 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -397,6 +397,29 @@ static inline int bio_iov_vecs_to_alloc(struct iov_iter *iter, int max_segs)
 	return iov_iter_npages(iter, max_segs);
 }
 
+/**
+ * bio_iov_bounce_nr_vecs - calculate number of bvecs for a bounce bio
+ * @iter:	iter to bounce from
+ * @op:		REQ_OP_* for the bio
+ *
+ * Calculates how many bvecs are needed for the next bio to bounce from/to
+ * @iter.
+ */
+static inline unsigned short
+bio_iov_bounce_nr_vecs(struct iov_iter *iter, blk_opf_t op)
+{
+	/*
+	 * We still need to bounce bvec iters, so don't special case them
+	 * here unlike in bio_iov_vecs_to_alloc.
+	 *
+	 * For reads we need to use a vector for the bounce buffer, account
+	 * for that here.
+	 */
+	if (op_is_write(op))
+		return iov_iter_npages(iter, BIO_MAX_VECS);
+	return iov_iter_npages(iter, BIO_MAX_VECS - 1) + 1;
+}
+
 struct request_queue;
 
 void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
@@ -450,6 +473,9 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty);
 extern void bio_set_pages_dirty(struct bio *bio);
 extern void bio_check_pages_dirty(struct bio *bio);
 
+int bio_iov_iter_bounce(struct bio *bio, struct iov_iter *iter);
+void bio_iov_iter_unbounce(struct bio *bio, bool is_error, bool mark_dirty);
+
 extern void bio_copy_data_iter(struct bio *dst, struct bvec_iter *dst_iter,
 			       struct bio *src, struct bvec_iter *src_iter);
 extern void bio_copy_data(struct bio *dst, struct bio *src);