In my test, 128M memory is hot added, but zone's pcp batch is 0, which is

an obvious error.  When pages are onlined, zone pcp should be updated
accordingly.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Yakui Zhao <yakui.zhao@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1065,6 +1065,8 @@ extern void setup_per_cpu_pageset(void);
 static inline void setup_per_cpu_pageset(void) {}
 #endif
 
+extern void zone_pcp_update(struct zone *zone);
+
 /* nommu.c */
 extern atomic_long_t mmap_pages_allocated;
 

mm/memory_hotplug.c

@@ -422,6 +422,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages)
 	zone->present_pages += onlined_pages;
 	zone->zone_pgdat->node_present_pages += onlined_pages;
 
+	zone_pcp_update(zone);
 	setup_per_zone_wmarks();
 	calculate_zone_inactive_ratio(zone);
 	if (onlined_pages) {

mm/page_alloc.c

@@ -3158,6 +3158,31 @@ int zone_wait_table_init(struct zone *zone, unsigned long zone_size_pages)
 	return 0;
 }
 
+static int __zone_pcp_update(void *data)
+{
+	struct zone *zone = data;
+	int cpu;
+	unsigned long batch = zone_batchsize(zone), flags;
+
+	for (cpu = 0; cpu < NR_CPUS; cpu++) {
+		struct per_cpu_pageset *pset;
+		struct per_cpu_pages *pcp;
+
+		pset = zone_pcp(zone, cpu);
+		pcp = &pset->pcp;
+
+		local_irq_save(flags);
+		free_pages_bulk(zone, pcp->count, &pcp->list, 0);
+		setup_pageset(pset, batch);
+		local_irq_restore(flags);
+	}
+}
+
+void zone_pcp_update(struct zone *zone)
+{
+	stop_machine(__zone_pcp_update, zone, NULL);
+}
+
 static __meminit void zone_pcp_init(struct zone *zone)
 {
 	int cpu;