block: make blk_rq_map_user take a NULL user-space buffer

This patch changes blk_rq_map_user to accept a NULL user-space buffer
with a READ command if rq_map_data is not NULL. Thus a caller can pass
page frames to lk_rq_map_user to just set up a request and bios with
page frames propely. bio_uncopy_user (called via blk_rq_unmap_user)
doesn't copy data to user space with such request.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index bc386cd5e996..7af373f253dc 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -108,6 +108,7 @@ struct bio {
 #define BIO_USER_MAPPED 6       /* contains user pages */
 #define BIO_EOPNOTSUPP  7       /* not supported */
 #define BIO_CPU_AFFINE  8       /* complete bio on same CPU as submitted */
+#define BIO_NULL_MAPPED 9       /* contains invalid user pages */
 #define bio_flagged(bio, flag)  ((bio)->bi_flags & (1 << (flag)))
 
 /*