mm/page_alloc: simplify update of pgdat in wake_all_kswapds

There is no need to update last_pgdat for each zone, only update
last_pgdat when iterating the first zone of a node.

Link: https://lkml.kernel.org/r/20220322115635.2708989-1-chenwandun@huawei.comSigned-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_alloc.c

@@ -4631,9 +4631,10 @@ static void wake_all_kswapds(unsigned int order, gfp_t gfp_mask,
 					ac->nodemask) {
 		if (!managed_zone(zone))
 			continue;
-		if (last_pgdat != zone->zone_pgdat)
+		if (last_pgdat != zone->zone_pgdat) {
 			wakeup_kswapd(zone, gfp_mask, order, highest_zoneidx);
-		last_pgdat = zone->zone_pgdat;
+			last_pgdat = zone->zone_pgdat;
+		}
 	}
 }