block: clear drain buffer if draining for write command

Clear drain buffer before chaining if the command in question is a
write.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index bef1b4d0fc02..7506c4fe0264 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -221,6 +221,9 @@ new_segment:
         } /* segments in rq */
 
         if (q->dma_drain_size && q->dma_drain_needed(rq)) {
+                if (rq->cmd_flags & REQ_RW)
+                        memset(q->dma_drain_buffer, 0, q->dma_drain_size);
+
                 sg->page_link &= ~0x02;
                 sg = sg_next(sg);
                 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),