Subject: clear compound mapping

From: Andrea Arcangeli <aarcange@redhat.com>

Clear compound mapping for anonymous compound pages like it already happens for
regular anonymous pages. But crash if mapping is set for any tail page, also
the PageAnon check is meaningless for tail pages. This check only makes sense
for the head page, for tail page it can only hide bugs and we definitely don't
want to hide bugs.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
---

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -644,13 +644,10 @@ static bool free_pages_prepare(struct pa
 	trace_mm_page_free_direct(page, order);
 	kmemcheck_free_shadow(page, order);
 
-	for (i = 0; i < (1 << order); i++) {
-		struct page *pg = page + i;
-
-		if (PageAnon(pg))
-			pg->mapping = NULL;
-		bad += free_pages_check(pg);
-	}
+	if (PageAnon(page))
+		page->mapping = NULL;
+	for (i = 0; i < (1 << order); i++)
+		bad += free_pages_check(page + i);
 	if (bad)
 		return false;