[PATCH v2 1/8] drm: Add dummy page per device or GEM object

[Daniel Vetter]

On Mon, Jun 22, 2020 at 4:22 PM Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> On Mon, 22 Jun 2020 11:35:01 +0200
> Daniel Vetter <daniel at ffwll.ch> wrote:
>
> > On Sun, Jun 21, 2020 at 02:03:01AM -0400, Andrey Grodzovsky wrote:
> > > Will be used to reroute CPU mapped BO's page faults once
> > > device is removed.
> > >
> > > Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
> > > ---
> > >  drivers/gpu/drm/drm_file.c  |  8 ++++++++
> > >  drivers/gpu/drm/drm_prime.c | 10 ++++++++++
> > >  include/drm/drm_file.h      |  2 ++
> > >  include/drm/drm_gem.h       |  2 ++
> > >  4 files changed, 22 insertions(+)
>
> ...
>
> > > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > > index 0b37506..47460d1 100644
> > > --- a/include/drm/drm_gem.h
> > > +++ b/include/drm/drm_gem.h
> > > @@ -310,6 +310,8 @@ struct drm_gem_object {
> > >      *
> > >      */
> > >     const struct drm_gem_object_funcs *funcs;
> > > +
> > > +   struct page *dummy_page;
> > >  };
> >
> > I think amdgpu doesn't care, but everyone else still might care somewhat
> > about flink. That also shares buffers, so also needs to allocate the
> > per-bo dummy page.
>
> Do you really care about making flink not explode on device
> hot-unplug? Why not just leave flink users die in a fire?
> It's not a regression.

It's not about exploding, they won't. With flink you can pass a buffer
from one address space to the other, so imo we should avoid false
sharing. E.g. if you happen to write something $secret into a private
buffer, but only $non-secret stuff into shared buffers. Then if you
unplug, your well-kept $secret might suddenly be visible by lots of
other processes you never intended to share it with.

Just feels safer to plug that hole completely.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch