quoted

-	if (dio->end_io && dio->result)
-		dio->end_io(dio->iocb, offset, transferred, dio->private);
+	if (dio->end_io)
+		dio->end_io(dio->iocb, offset, ret, dio->private);

Could we make end_io return an int so that errors during completion can be
stuffed into ret to be picked up by whatever's calling directio?  Something
like this:

if (dio->end_io) {
	int ret2;

	ret2 = dio->end_io(dio->iocb, offset, ret, dio->private);
	if (ret2 && !ret)
		ret = ret2;
}

That way I can capture IO errors during the CoW remapping step and pass them to
userland either via dio_complete()'s return value or through ki_complete.

(If ret itself is an error code then obviously we don't bother with the
post-CoW remap.)

Should be doable, I'll respin it with that change.