block: share request flush fields with elevator_private

Flush requests are never put on the IO scheduler.  Convert request
structure's elevator_private* into an array and have the flush fields
share a union with it.

Reclaim the space lost in 'struct request' by moving 'completion_data'
back in the union with 'rb_node'.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 12 insertions, 11 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 6d7e9afd08c3..12bb426949e9 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -99,25 +99,26 @@ struct request {
         /*
          * The rb_node is only used inside the io scheduler, requests
          * are pruned when moved to the dispatch queue. So let the
-         * flush fields share space with the rb_node.
+         * completion_data share space with the rb_node.
          */
         union {
                 struct rb_node rb_node; /* sort/lookup */
-                struct {
-                        unsigned int                    seq;
-                        struct list_head                list;
-                } flush;
+                void *completion_data;
         };
 
-        void *completion_data;
-
         /*
          * Three pointers are available for the IO schedulers, if they need
-         * more they have to dynamically allocate it.
+         * more they have to dynamically allocate it.  Flush requests are
+         * never put on the IO scheduler. So let the flush fields share
+         * space with the three elevator_private pointers.
          */
-        void *elevator_private;
-        void *elevator_private2;
-        void *elevator_private3;
+        union {
+                void *elevator_private[3];
+                struct {
+                        unsigned int            seq;
+                        struct list_head        list;
+                } flush;
+        };
 
         struct gendisk *rq_disk;
         unsigned long start_time;