thp: put_page: recheck PageHead after releasing the compound_lock

After releasing the compound_lock split_huge_page can still run and release the
page before put_page_testzero runs.
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap.c

@@ -131,8 +131,12 @@ static void put_compound_page(struct page *page)
 			atomic_dec(&page->_count);
 			VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
 			compound_unlock_irqrestore(page_head, flags);
-			if (put_page_testzero(page_head))
-				__put_compound_page(page_head);
+			if (put_page_testzero(page_head)) {
+				if (PageHead(page_head))
+					__put_compound_page(page_head);
+				else
+					__put_single_page(page_head);
+			}
 		} else {
 			/* page_head is a dangling pointer */
 			VM_BUG_ON(PageTail(page));