diff --git a/mm/vmstat.c b/mm/vmstat.c
index c823776..f42745e 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -493,6 +493,10 @@
 			atomic_long_add(global_diff[i], &vm_stat[i]);
 }
 
+/*
+ * this is only called if !populated_zone(zone), which implies no other users of
+ * pset->vm_stat_diff[] exsist.
+ */
 void drain_zonestat(struct zone *zone, struct per_cpu_pageset *pset)
 {
 	int i;