mm, page_alloc: check once if a zone has isolated pageblocks

When bulk freeing pages from the per-cpu lists the zone is checked for
isolated pageblocks on every release.  This patch checks it once per
drain.

[mgorman@techsingularity.net: fix locking radce, per Vlastimil]
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -831,8 +831,10 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 	int batch_free = 0;
 	int to_free = count;
 	unsigned long nr_scanned;
+	bool isolated_pageblocks;
 
 	spin_lock(&zone->lock);
+	isolated_pageblocks = has_isolate_pageblock(zone);
 	nr_scanned = zone_page_state(zone, NR_PAGES_SCANNED);
 	if (nr_scanned)
 		__mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);
@@ -870,7 +872,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
 			/* MIGRATE_ISOLATE page should not go to pcplists */
 			VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
 			/* Pageblock could have been isolated meanwhile */
-			if (unlikely(has_isolate_pageblock(zone)))
+			if (unlikely(isolated_pageblocks))
 				mt = get_pageblock_migratetype(page);
 
 			__free_one_page(page, page_to_pfn(page), zone, 0, mt);