swap: redirty page if page write fails on swap file

Since commit 62c230b ("mm: add support for a filesystem to activate
swap files and use direct_IO for writing swap pages"), swap_writepage()
calls direct_IO on swap files.  However, in that case the page isn't
redirtied if I/O fails, and is therefore handled afterwards as if it has
been successfully written to the swap file, leading to memory corruption
when the page is eventually swapped back in.

This patch sets the page dirty when direct_IO() fails.  It fixes a
memory corruption that happened while using swap-over-NFS.

Signed-off-by: Jerome Marchand <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: <[email protected]>	[3.6+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_io.c

@@ -231,6 +231,8 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		if (ret == PAGE_SIZE) {
 			count_vm_event(PSWPOUT);
 			ret = 0;
+		} else {
+			set_page_dirty(page);
 		}
 		return ret;
 	}