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, 4 insertions, 4 deletions

diff --git a/fs/bio.c b/fs/bio.c
index 9d68ddb89b71..355302985e22 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -547,11 +547,11 @@ static int __bio_copy_iov(struct bio *bio, struct bio_vec *iovecs,
 int bio_uncopy_user(struct bio *bio)
 {
         struct bio_map_data *bmd = bio->bi_private;
-        int ret;
-
-        ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs, bmd->nr_sgvecs, 1,
-                             bmd->is_our_pages);
+        int ret = 0;
 
+        if (!bio_flagged(bio, BIO_NULL_MAPPED))
+                ret = __bio_copy_iov(bio, bmd->iovecs, bmd->sgvecs,
+                                     bmd->nr_sgvecs, 1, bmd->is_our_pages);
         bio_free_map_data(bmd);
         bio_put(bio);
         return ret;