Simplify elevator algorithm, make it prefer reads heavily.

This is needed for reasonable read latency with the new VM
behaviour. 

NOTE! This is way too unfair, Andrew and Jens are working on
alternatives.

drivers/block/elevator.c

@@ -157,13 +157,20 @@ inline int elv_try_last_merge(request_queue_t *q, struct request **req,
 	return ret;
 }
 
+static int bio_rq_before(struct bio *bio, struct request *rq)
+{
+	if (!kdev_same(to_kdev_t(bio->bi_bdev->bd_dev), rq->rq_dev))
+		return 0;
+	return bio->bi_sector < rq->sector;
+}
+
 /*
  * elevator_linux starts here
  */
 int elevator_linus_merge(request_queue_t *q, struct request **req,
 			 struct bio *bio)
 {
-	struct list_head *entry;
+	struct list_head *entry, *good;
 	struct request *__rq;
 	int ret;
 
@@ -171,6 +178,7 @@ int elevator_linus_merge(request_queue_t *q, struct request **req,
 		return ret;
 
 	entry = &q->queue_head;
+	good = &q->queue_head;
 	ret = ELEVATOR_NO_MERGE;
 	while ((entry = entry->prev) != &q->queue_head) {
 		__rq = list_entry_rq(entry);
@@ -178,37 +186,31 @@ int elevator_linus_merge(request_queue_t *q, struct request **req,
 		if (__rq->flags & (REQ_BARRIER | REQ_STARTED))
 			break;
 		if (!(__rq->flags & REQ_CMD))
-			continue;
-
-		if (elv_linus_sequence(__rq) < bio_sectors(bio))
 			break;
 
-		if (!*req && bio_rq_in_between(bio, __rq, &q->queue_head))
-			*req = __rq;
+		if (bio_data_dir(bio) != rq_data_dir(__rq)) {
+			if (bio_data_dir(bio) == WRITE)
+				break;
+			good = entry->prev;
+			continue;
+		}
 
-		if ((ret = elv_try_merge(__rq, bio))) {
-			if (ret == ELEVATOR_FRONT_MERGE)
-				elv_linus_sequence(__rq) -= bio_sectors(bio);
+		ret = elv_try_merge(__rq, bio);
+		if (ret) {
 			*req = __rq;
 			q->last_merge = &__rq->queuelist;
-			break;
-		}
+			return ret;
 		}
 
-	/*
-	 * if *req, it's either a seek or merge in the middle of the queue
-	 */
-	if (*req) {
-		struct list_head *entry = &(*req)->queuelist;
-		int cost = ret ? 1 : ELV_LINUS_SEEK_COST;
+		if (bio_rq_before(bio, __rq))
+			good = entry->prev;
 
-		while ((entry = entry->next) != &q->queue_head) {
-			__rq = list_entry_rq(entry);
-			elv_linus_sequence(__rq) -= cost;
-		}
 	}
 
-	return ret;
+	if (good != &q->queue_head)
+		*req = list_entry_rq(good);
+
+	return ELEVATOR_NO_MERGE;
 }
 
 void elevator_linus_merge_req(request_queue_t *q, struct request *req,