mm/memory-failure: fix race with compound page split/merge

get_hwpoison_page() must recheck relation between head and tail pages.

n-horiguchi said: without this recheck, the race causes kernel to pin an
irrelevant page, and finally makes kernel crash for refcount mismatch.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory-failure.c

@@ -888,7 +888,15 @@ int get_hwpoison_page(struct page *page)
 		}
 	}
 
-	return get_page_unless_zero(head);
+	if (get_page_unless_zero(head)) {
+		if (head == compound_head(page))
+			return 1;
+
+		pr_info("MCE: %#lx cannot catch tail\n", page_to_pfn(page));
+		put_page(head);
+	}
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(get_hwpoison_page);