block: Initial support for data-less (or empty) barrier support

This implements functionality to pass down or insert a barrier
in a queue, without having data attached to it. The ->prepare_flush_fn()
infrastructure from data barriers are reused to provide this
functionality.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/mm/bounce.c b/mm/bounce.c
index 3b549bf31f7d..b6d2d0f1019b 100644
--- a/mm/bounce.c
+++ b/mm/bounce.c
@@ -265,6 +265,12 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
         mempool_t *pool;
 
         /*
+         * Data-less bio, nothing to bounce
+         */
+        if (bio_empty_barrier(*bio_orig))
+                return;
+
+        /*
          * for non-isa bounce case, just check if the bounce pfn is equal
          * to or bigger than the highest pfn in the system -- in that case,
          * don't waste time iterating over bio segments