diff -puN fs/ext4/inode.c~debug-dont-prefault-on-write fs/ext4/inode.c
--- a/fs/ext4/inode.c~debug-dont-prefault-on-write	2015-10-05 10:12:35.286153137 -0700
+++ b/fs/ext4/inode.c	2015-10-05 10:50:37.372253050 -0700
@@ -1204,10 +1204,20 @@ static int ext4_journalled_write_end(str
 		copied = ext4_write_inline_data_end(inode, pos, len,
 						    copied, page);
 	else {
+		/*
+		 * With a short write (copied < len) we have potentially
+		 * valuable data in 'page'.  But, when the page is made Uptodate
+		 * this data will be overwritten.  Setting copied=0 will tell
+		 * the upper layers to repeat the write to 'page'.
+		 *
+		 * Only bother zeroing out buffers when we have _actually_
+		 * written data.
+		 */
 		if (copied < len) {
 			if (!PageUptodate(page))
 				copied = 0;
-			page_zero_new_buffers(page, from+copied, to);
+			if (copied)
+				page_zero_new_buffers(page, from+copied, to);
 		}
 
 		ret = ext4_walk_page_buffers(handle, page_buffers(page), from,
_