mem-hotplug: reset node present pages when hot-adding a new pgdat

When memory is hot-added, all the memory is in offline state.  So clear
all zones' present_pages because they will be updated in online_pages()
and offline_pages().  Otherwise, /proc/zoneinfo will corrupt:

When the memory of node2 is offline:

  # cat /proc/zoneinfo
  ......
  Node 2, zone   Movable
  ......
        spanned  8388608
        present  8388608
        managed  0

When we online memory on node2:

  # cat /proc/zoneinfo
  ......
  Node 2, zone   Movable
  ......
        spanned  8388608
        present  16777216
        managed  8388608
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>	[3.16+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -1067,6 +1067,16 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
 }
 #endif /* CONFIG_MEMORY_HOTPLUG_SPARSE */
 
+static void reset_node_present_pages(pg_data_t *pgdat)
+{
+	struct zone *z;
+
+	for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
+		z->present_pages = 0;
+
+	pgdat->node_present_pages = 0;
+}
+
 /* we are OK calling __meminit stuff here - we have CONFIG_MEMORY_HOTPLUG */
 static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
 {
@@ -1105,6 +1115,13 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
 	 */
 	reset_node_managed_pages(pgdat);
 
+	/*
+	 * When memory is hot-added, all the memory is in offline state. So
+	 * clear all zones' present_pages because they will be updated in
+	 * online_pages() and offline_pages().
+	 */
+	reset_node_present_pages(pgdat);
+
 	return pgdat;
 }