block: implement request_queue->dma_drain_needed

Draining shouldn't be done for commands where overflow may indicate
data integrity issues.  Add dma_drain_needed callback to
request_queue.  Drain buffer is appened iff this function returns
non-zero.

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, 1 insertions, 1 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 39f2e077a014..bef1b4d0fc02 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -220,7 +220,7 @@ new_segment:
                 bvprv = bvec;
         } /* segments in rq */
 
-        if (q->dma_drain_size) {
+        if (q->dma_drain_size && q->dma_drain_needed(rq)) {
                 sg->page_link &= ~0x02;
                 sg = sg_next(sg);
                 sg_set_page(sg, virt_to_page(q->dma_drain_buffer),