[PATCH v2 2/2] block: share request flush fields with elevator_private

[Mike Snitzer]

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@xxxxxxxxxx>
---
 block/cfq-iosched.c    |   18 +++++++++---------
 block/elevator.c       |    2 +-
 include/linux/blkdev.h |   23 ++++++++++++-----------
 3 files changed, 22 insertions(+), 21 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 501ffdf..8692958 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -54,9 +54,9 @@ static const int cfq_hist_divisor = 4;
 #define CFQQ_SEEKY(cfqq)	(hweight32(cfqq->seek_history) > 32/8)
 
 #define RQ_CIC(rq)		\
-	((struct cfq_io_context *) (rq)->elevator_private)
-#define RQ_CFQQ(rq)		(struct cfq_queue *) ((rq)->elevator_private2)
-#define RQ_CFQG(rq)		(struct cfq_group *) ((rq)->elevator_private3)
+	((struct cfq_io_context *) (rq)->elevator_private[0])
+#define RQ_CFQQ(rq)		(struct cfq_queue *) ((rq)->elevator_private[1])
+#define RQ_CFQG(rq)		(struct cfq_group *) ((rq)->elevator_private[2])
 
 static struct kmem_cache *cfq_pool;
 static struct kmem_cache *cfq_ioc_pool;
@@ -3609,12 +3609,12 @@ static void cfq_put_request(struct request *rq)
 
 		put_io_context(RQ_CIC(rq)->ioc);
 
-		rq->elevator_private = NULL;
-		rq->elevator_private2 = NULL;
+		rq->elevator_private[0] = NULL;
+		rq->elevator_private[1] = NULL;
 
 		/* Put down rq reference on cfqg */
 		cfq_put_cfqg(RQ_CFQG(rq));
-		rq->elevator_private3 = NULL;
+		rq->elevator_private[2] = NULL;
 
 		cfq_put_queue(cfqq);
 	}
@@ -3702,9 +3702,9 @@ new_queue:
 
 	cfqq->allocated[rw]++;
 	cfqq->ref++;
-	rq->elevator_private = cic;
-	rq->elevator_private2 = cfqq;
-	rq->elevator_private3 = cfq_ref_get_cfqg(cfqq->cfqg);
+	rq->elevator_private[0] = cic;
+	rq->elevator_private[1] = cfqq;
+	rq->elevator_private[2] = cfq_ref_get_cfqg(cfqq->cfqg);
 
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
diff --git a/block/elevator.c b/block/elevator.c
index 270e097..f98e92e 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -764,7 +764,7 @@ int elv_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
 	if (e->ops->elevator_set_req_fn)
 		return e->ops->elevator_set_req_fn(q, rq, gfp_mask);
 
-	rq->elevator_private = NULL;
+	rq->elevator_private[0] = NULL;
 	return 0;
 }
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 8a082a5..e3ee74f 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;
 	struct hd_struct *part;
-- 
1.7.3.4

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