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>

1 files changed, 12 insertions, 0 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 3b0cd4249671..eafc94f68d79 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -362,6 +362,18 @@ static int attempt_merge(struct request_queue *q, struct request *req,
                 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
          */
         if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))