mm/vmstat: add note on safety of drain_zonestat

Signed-off-by: NCody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -493,6 +493,10 @@ void refresh_cpu_vm_stats(int cpu)
 			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;