fs/proc/page.c: add PageAnon check to surely detect thp

stable_page_flags() checks !PageHuge && PageTransCompound && PageLRU to
know that a specified page is thp or not.  But sometimes it's not enough
and we fail to detect thp when the thp is on pagevec.  This happens only
for a few seconds after LRU list operations, but it makes it difficult
to control our applications depending on this flag.

So this patch adds another check PageAnon to detect thps on pagevec.  It
might not give the future extensibility for thp pagecache, but it's OK
at least for now.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/page.c

@@ -118,10 +118,12 @@ u64 stable_page_flags(struct page *page)
 	/*
 	 * PageTransCompound can be true for non-huge compound pages (slab
 	 * pages or pages allocated by drivers with __GFP_COMP) because it
-	 * just checks PG_head/PG_tail, so we need to check PageLRU to make
-	 * sure a given page is a thp, not a non-huge compound page.
+	 * just checks PG_head/PG_tail, so we need to check PageLRU/PageAnon
+	 * to make sure a given page is a thp, not a non-huge compound page.
 	 */
-	else if (PageTransCompound(page) && PageLRU(compound_trans_head(page)))
+	else if (PageTransCompound(page) &&
+		 (PageLRU(compound_trans_head(page)) ||
+		  PageAnon(compound_trans_head(page))))
 		u |= 1 << KPF_THP;
 
 	/*