cifs: Strange O_DIRECT behaviour on non-directio mounts

[Kevin Wolf <kwolf-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Hi Steve,

I just discussed a problem with a qemu user on IRC, which boiled down to
him trying to open an image file on cifs with O_DIRECT, but not using a
directio mount. I understand that this probably isn't going to work
anytime soon (if at all), but it resulted in a rather unhelpful failure
mode.

What happens is that cifs lets the open() call succeed even with the
unsupported O_DIRECT on that mount, but then fails any I/O on the file
descriptor. I believe this was introduced in commit dca69288 (which I
think is otherwise pretty useful).

With the old behaviour, qemu detected what's going on and suggested to
use a non-O_DIRECT mode to the user, but with the new one, it got rather
unhappy after failing to find a working O_DIRECT alignment and ran into
an assertion failure...

Now I'll certainly fix the latter in qemu, but I also think that the
behaviour of cifs is rather surprising. Any chance that you can make
open() with O_DIRECT fail again on non-directio mounts?

Thanks,
Kevin