diff --git a/mm/ksm.c b/mm/ksm.c
index dbfb8f98b189..9c51925825cb 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -475,6 +475,7 @@ static void advisor_stop_scan(void)
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
+/* Number of allocated stable and unstable tree roots. */
 static int ksm_nr_node_ids = 1;
 #else
 #define ksm_merge_across_nodes	1U
@@ -3636,14 +3637,13 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
 			else {
 				root_stable_tree = buf;
 				root_unstable_tree = buf + nr_node_ids;
+				ksm_nr_node_ids = nr_node_ids;
 				/* Stable tree is empty but not the unstable */
 				root_unstable_tree[0] = one_unstable_tree[0];
 			}
 		}
-		if (!err) {
+		if (!err)
 			ksm_merge_across_nodes = knob;
-			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
-		}
 	}
 	mutex_unlock(&ksm_thread_mutex);