mmzone: note that node_size_lock should be manipulated via pgdat_resize_lock()

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -733,6 +733,9 @@ typedef struct pglist_data {
 	 * or node_spanned_pages stay constant.  Holding this will also
 	 * guarantee that any pfn_valid() stays that way.
 	 *
+	 * pgdat_resize_lock() and pgdat_resize_unlock() are provided to
+	 * manipulate node_size_lock without checking for CONFIG_MEMORY_HOTPLUG.
+	 *
 	 * Nests above zone->lock and zone->span_seqlock
 	 */
 	spinlock_t node_size_lock;