diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ea9da3b..886ea0d 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -824,6 +824,7 @@
 		mapping2 = page_mapping(page);
 		if (mapping2) { /* Race with truncate? */
 			BUG_ON(mapping2 != mapping);
+			WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
 			if (mapping_cap_account_dirty(mapping)) {
 				__inc_zone_page_state(page, NR_FILE_DIRTY);
 				task_io_account_write(PAGE_CACHE_SIZE);