1 files changed, 44 insertions, 7 deletions

diff --git a/block/elevator.c b/block/elevator.c
index ac7c8ad580ba..520d6b224b74 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -651,9 +651,46 @@ static inline bool elv_support_iosched(struct request_queue *q)
 }
 
 /*
- * For blk-mq devices supporting IO scheduling, we default to using mq-deadline,
- * if available, for single queue devices. If deadline isn't available OR
- * deadline initialization fails OR we have multiple queues, default to "none".
+ * For single queue devices, default to using mq-deadline. If we have multiple
+ * queues or mq-deadline is not available, default to "none".
+ */
+static struct elevator_type *elevator_get_default(struct request_queue *q)
+{
+	if (q->nr_hw_queues != 1)
+		return NULL;
+
+	return elevator_get(q, "mq-deadline", false);
+}
+
+/*
+ * Get the first elevator providing the features required by the request queue.
+ * Default to "none" if no matching elevator is found.
+ */
+static struct elevator_type *elevator_get_by_features(struct request_queue *q)
+{
+	struct elevator_type *e;
+
+	spin_lock(&elv_list_lock);
+
+	list_for_each_entry(e, &elv_list, list) {
+		if (elv_support_features(e->elevator_features,
+					 q->required_elevator_features))
+			break;
+	}
+
+	if (e && !try_module_get(e->elevator_owner))
+		e = NULL;
+
+	spin_unlock(&elv_list_lock);
+
+	return e;
+}
+
+/*
+ * For a device queue that has no required features, use the default elevator
+ * settings. Otherwise, use the first elevator available matching the required
+ * features. If no suitable elevator is find or if the chosen elevator
+ * initialization fails, fall back to the "none" elevator (no elevator).
  */
 void elevator_init_mq(struct request_queue *q)
 {
@@ -663,15 +700,15 @@ void elevator_init_mq(struct request_queue *q)
 	if (!elv_support_iosched(q))
 		return;
 
-	if (q->nr_hw_queues != 1)
-		return;
-
 	WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
 
 	if (unlikely(q->elevator))
 		return;
 
-	e = elevator_get(q, "mq-deadline", false);
+	if (!q->required_elevator_features)
+		e = elevator_get_default(q);
+	else
+		e = elevator_get_by_features(q);
 	if (!e)
 		return;