mm/filemap.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 72940fb38666..e8d01936817a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2493,6 +2493,20 @@ again:
 		pagefault_enable();
 		flush_dcache_page(page);
 
+		/*
+		 * If we didn't successfully copy all the data from user space,
+		 * and the target page is not up-to-date, we will have to prefault
+		 * the source. And if the page wasn't up-to-date before the write,
+		 * the "write_end()" may need to *make* it up-to-date, and thus
+		 * overwrite our partial copy.
+		 *
+		 * So for that case, thow away the whole thing and force a full
+		 * restart (see comment above, and iov_iter_fault_in_readable()
+		 * below).
+		 */
+		if (copied < bytes && !PageUptodate(page))
+			copied = 0;
+
 		status = a_ops->write_end(file, mapping, pos, bytes, copied,
 						page, fsdata);
 		if (unlikely(status < 0))