cifs: writeback fix

Wait for the page to be written to the cache before we allow it
to be modified

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/file.c

@@ -4210,13 +4210,19 @@ cifs_page_mkwrite(struct vm_fault *vmf)
 {
 	struct page *page = vmf->page;
 
+	/* Wait for the page to be written to the cache before we allow it to
+	 * be modified.  We then assume the entire page will need writing back.
+	 */
 #ifdef CONFIG_CIFS_FSCACHE
 	if (PageFsCache(page) &&
 	    wait_on_page_fscache_killable(page) < 0)
 		return VM_FAULT_RETRY;
 #endif
 
-	lock_page(page);
+	wait_on_page_writeback(page);
+
+	if (lock_page_killable(page) < 0)
+		return VM_FAULT_RETRY;
 	return VM_FAULT_LOCKED;
 }