mm/memory_hotplug: just build zonelist for newly added node

Commit 9adb62a5 ("mm/hotplug: correctly setup fallback zonelists
when creating new pgdat") tries to build the correct zonelist for a
newly added node, while it is not necessary to rebuild it for already
exist nodes.

In build_zonelists(), it will iterate on nodes with memory.  For a newly
added node, it will have memory until node_states_set_node() is called
in online_pages().

This patch avoids rebuilding the zonelists for already existing nodes.

build_zonelists_node() uses managed_zone(zone) checks, so it should not
include empty zones anyway.  So effectively we avoid some pointless work
under stop_machine().

[akpm@linux-foundation.org: tweak comment text]
[akpm@linux-foundation.org: coding-style tweak, per Vlastimil]
Link: http://lkml.kernel.org/r/20170626035822.50155-1-richard.weiyang@gmail.comSigned-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -5282,14 +5282,18 @@ static int __build_all_zonelists(void *data)
 	memset(node_load, 0, sizeof(node_load));
 #endif
 
+	/*
+	 * This node is hotadded and no memory is yet present.   So just
+	 * building zonelists is fine - no need to touch other nodes.
+	 */
 	if (self && !node_online(self->node_id)) {
 		build_zonelists(self);
-	}
-
-	for_each_online_node(nid) {
-		pg_data_t *pgdat = NODE_DATA(nid);
+	} else {
+		for_each_online_node(nid) {
+			pg_data_t *pgdat = NODE_DATA(nid);
 
-		build_zonelists(pgdat);
+			build_zonelists(pgdat);
+		}
 	}
 
 	/*