3 files changed, 58 insertions, 4 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 533a2938ffd6..9fc5eafa6c0e 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -568,6 +568,38 @@ cfq_merged_requests(request_queue_t *q, struct request *rq,
         cfq_remove_request(next);
 }
 
+static int cfq_allow_merge(request_queue_t *q, struct request *rq,
+                           struct bio *bio)
+{
+        struct cfq_data *cfqd = q->elevator->elevator_data;
+        const int rw = bio_data_dir(bio);
+        struct cfq_queue *cfqq;
+        pid_t key;
+
+        /*
+         * If bio is async or a write, always allow merge
+         */
+        if (!bio_sync(bio) || rw == WRITE)
+                return 1;
+
+        /*
+         * bio is sync. if request is not, disallow.
+         */
+        if (!rq_is_sync(rq))
+                return 0;
+
+        /*
+         * Ok, both bio and request are sync. Allow merge if they are
+         * from the same queue.
+         */
+        key = cfq_queue_pid(current, rw, 1);
+        cfqq = cfq_find_cfq_hash(cfqd, key, current->ioprio);
+        if (cfqq != RQ_CFQQ(rq))
+                return 0;
+
+        return 1;
+}
+
 static inline void
 __cfq_set_active_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 {
@@ -2125,6 +2157,7 @@ static struct elevator_type iosched_cfq = {
                 .elevator_merge_fn =            cfq_merge,
                 .elevator_merged_fn =           cfq_merged_request,
                 .elevator_merge_req_fn =        cfq_merged_requests,
+                .elevator_allow_merge_fn =      cfq_allow_merge,
                 .elevator_dispatch_fn =         cfq_dispatch_requests,
                 .elevator_add_req_fn =          cfq_insert_request,
                 .elevator_activate_req_fn =     cfq_activate_request,
diff --git a/block/elevator.c b/block/elevator.c
index c0063f345c5d..62c7a3069d3a 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -51,6 +51,21 @@ static const int elv_hash_shift = 6;
 #define ELV_ON_HASH(rq)         (!hlist_unhashed(&(rq)->hash))
 
 /*
+ * Query io scheduler to see if the current process issuing bio may be
+ * merged with rq.
+ */
+static int elv_iosched_allow_merge(struct request *rq, struct bio *bio)
+{
+        request_queue_t *q = rq->q;
+        elevator_t *e = q->elevator;
+
+        if (e->ops->elevator_allow_merge_fn)
+                return e->ops->elevator_allow_merge_fn(q, rq, bio);
+
+        return 1;
+}
+
+/*
  * can we safely merge with this request?
  */
 inline int elv_rq_merge_ok(struct request *rq, struct bio *bio)
@@ -65,12 +80,15 @@ inline int elv_rq_merge_ok(struct request *rq, struct bio *bio)
                 return 0;
 
         /*
-         * same device and no special stuff set, merge is ok
+         * must be same device and not a special request
          */
-        if (rq->rq_disk == bio->bi_bdev->bd_disk && !rq->special)
-                return 1;
+        if (rq->rq_disk != bio->bi_bdev->bd_disk || !rq->special)
+                return 0;
 
-        return 0;
+        if (!elv_iosched_allow_merge(rq, bio))
+                return 0;
+
+        return 1;
 }
 EXPORT_SYMBOL(elv_rq_merge_ok);
 
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index a24931d24404..e88fcbc77f8f 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -12,6 +12,8 @@ typedef void (elevator_merge_req_fn) (request_queue_t *, struct request *, struc
 
 typedef void (elevator_merged_fn) (request_queue_t *, struct request *, int);
 
+typedef int (elevator_allow_merge_fn) (request_queue_t *, struct request *, struct bio *);
+
 typedef int (elevator_dispatch_fn) (request_queue_t *, int);
 
 typedef void (elevator_add_req_fn) (request_queue_t *, struct request *);
@@ -33,6 +35,7 @@ struct elevator_ops
         elevator_merge_fn *elevator_merge_fn;
         elevator_merged_fn *elevator_merged_fn;
         elevator_merge_req_fn *elevator_merge_req_fn;
+        elevator_allow_merge_fn *elevator_allow_merge_fn;
 
         elevator_dispatch_fn *elevator_dispatch_fn;
         elevator_add_req_fn *elevator_add_req_fn;