mm: make get_scan_ratio() safe for memcg

Currently, get_scan_ratio() always calculate the balancing value for
global reclaim and memcg reclaim doesn't use it.  Therefore it doesn't
have scan_global_lru() condition.

However, we plan to expand get_scan_ratio() to be usable for memcg too,
latter.  Then, The dependency code of global reclaim in the
get_scan_ratio() insert into scan_global_lru() condision explictly.

This patch doesn't have any functional change.
Acked-by: NRik van Riel <riel@redhat.com>
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1376,14 +1376,17 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
 		zone_nr_pages(zone, sc, LRU_INACTIVE_ANON);
 	file  = zone_nr_pages(zone, sc, LRU_ACTIVE_FILE) +
 		zone_nr_pages(zone, sc, LRU_INACTIVE_FILE);
-	free  = zone_page_state(zone, NR_FREE_PAGES);
 
-	/* If we have very few page cache pages, force-scan anon pages. */
+	if (scan_global_lru(sc)) {
+		free  = zone_page_state(zone, NR_FREE_PAGES);
+		/* If we have very few page cache pages,
+		   force-scan anon pages. */
 		if (unlikely(file + free <= zone->pages_high)) {
 			percent[0] = 100;
 			percent[1] = 0;
 			return;
 		}
+	}
 
 	/*
 	 * OK, so we have swap space and a fair amount of page cache