diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 1065d7c65fa1..9200e2aee746 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -81,6 +81,9 @@ int blk_mq_update_queue_map(unsigned int *map, unsigned int nr_queues)
 			map[i] = map[first_sibling];
 	}
 
+	for (i = 0; i < queue; i++)
+		printk(KERN_ERR "cpumap %d -> %d\n", i, map[i]);
+
 	free_cpumask_var(cpus);
 	return 0;
 }
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 68929bad9a6a..1678da3505ea 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1265,12 +1265,25 @@ run_queue:
 	blk_mq_put_ctx(data.ctx);
 }
 
+static int did_warn;
+
 /*
  * Default mapping to a software queue, since we use one per CPU.
  */
 struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *q, const int cpu)
 {
-	return q->queue_hw_ctx[q->mq_map[cpu]];
+	int i;
+
+	i = q->mq_map[cpu];
+	if (!i || did_warn)
+		return q->queue_hw_ctx[0];
+
+	printk(KERN_ERR "blk-mq: cpu %u got queue %u\n", cpu, i);
+	for_each_online_cpu(i)
+		printk(KERN_ERR "  cpu%d -> queue index %u\n", i, q->mq_map[i]);
+
+	did_warn = 1;
+	return q->queue_hw_ctx[0];
 }
 EXPORT_SYMBOL(blk_mq_map_queue);