Re: cifs: Strange O_DIRECT behaviour on non-directio mounts

[Steve French <smfrench-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

What is the server type?  (you can view this by seeing the "NOS" field
in /proc/fs/cifs/DebugData or checking to see if "unix" is listed in
the mount when you do "cat /proc/mounts | grep cifs")

When mounted to Samba for example, with cifs unix extensions
negotiated, the cifs client will send open flags (including O_DIRECT
IIRC) to the server on open.  When mounted to Windows/NetApp etc. or
when using "nounix" on mount, the flag is not sent on open.

On Fri, Jun 27, 2014 at 6:14 AM, Kevin Wolf <kwolf-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> 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



-- 
Thanks,

Steve