block: prevent merges of discard and write requests

Add logic to prevent two I/O requests being merged if
only one of them is a discard.  Ditto secure discard.

Without this fix, it is possible for write requests
to transform into discard requests.  For example:

  Submit bio 1 to discard 8 sectors from sector n
  Submit bio 2 to write 8 sectors from sector n + 16
  Submit bio 3 to write 8 sectors from sector n + 8

Bio 1 becomes request 1.  Bio 2 becomes request 2.
Bio 3 is merged with request 2, and then subsequently
request 2 is merged with request 1 resulting in just
one I/O request which discards all 24 sectors.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
(Moved the checks above the position checks /Jens)

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 3b0cd4249671d9826b42ea10473a11486c403368..eafc94f68d79f2ceb1c0a89fce8e449c3a5ff3f6 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -361,6 +361,18 @@ static int attempt_merge(struct request_queue *q, struct request *req,
        if (!rq_mergeable(req) || !rq_mergeable(next))
                return 0;
 
+       /*
+        * Don't merge file system requests and discard requests
+        */
+       if ((req->cmd_flags & REQ_DISCARD) != (next->cmd_flags & REQ_DISCARD))
+               return 0;
+
+       /*
+        * Don't merge discard requests and secure discard requests
+        */
+       if ((req->cmd_flags & REQ_SECURE) != (next->cmd_flags & REQ_SECURE))
+               return 0;
+
        /*
         * not contiguous
         */