block: update bio according to DMA alignment padding

DMA start address and transfer size alignment for PC requests are
achieved using bio_copy_user() instead of bio_map_user().  This works
because bio_copy_user() always uses full pages and block DMA alignment
isn't allowed to go over PAGE_SIZE.

However, the implementation didn't update the last bio of the request
to make this padding visible to lower layers.  This patch makes
blk_rq_map_user() extend the last bio such that it includes the
padding area and the size of area pointed to by the request is
properly aligned.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/block/blk-map.c b/block/blk-map.c
index bc5ce60691c3..a7cf63ccb5cf 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -139,6 +139,23 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
                 ubuf += ret;
         }
 
+        /*
+         * __blk_rq_map_user() copies the buffers if starting address
+         * or length isn't aligned.  As the copied buffer is always
+         * page aligned, we know that there's enough room for padding.
+         * Extend the last bio and update rq->data_len accordingly.
+         *
+         * On unmap, bio_uncopy_user() will use unmodified
+         * bio_map_data pointed to by bio->bi_private.
+         */
+        if (len & queue_dma_alignment(q)) {
+                unsigned int pad_len = (queue_dma_alignment(q) & ~len) + 1;
+                struct bio *bio = rq->biotail;
+
+                bio->bi_io_vec[bio->bi_vcnt - 1].bv_len += pad_len;
+                bio->bi_size += pad_len;
+        }
+
         rq->buffer = rq->data = NULL;
         return 0;
 unmap_rq: