[PATCH] truncate: dirty memory accounting fix

Only (un)account for IO and page-dirtying for devices which have real backing
store (ie: not tmpfs or ramdisks).

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/truncate.c

@@ -60,7 +60,8 @@ void cancel_dirty_page(struct page *page, unsigned int account_size)
 		WARN_ON(++warncount < 5);
 	}
 
-	if (TestClearPageDirty(page) && account_size) {
+	if (TestClearPageDirty(page) && account_size &&
+			mapping_cap_account_dirty(page->mapping)) {
 		dec_zone_page_state(page, NR_FILE_DIRTY);
 		task_io_account_cancelled_write(account_size);
 	}