[PATCH v2] drm/gem: Fix mmap fake offset handling for drm_gem_object_funcs.mmap

[Gerd Hoffmann]

> You need memory pressure, to force ttm to unmap the bo, not userspace. So
> roughly
> 1. create bo
> 2. mmap it through drm fd, write some stuff
> 3. export to dma-buf, mmap it, verify stuff is there
> 4. create a pile more bo, mmap them write to them
> 5. once you've thrashed all of vram enough, recheck your original bo. If
> I'm right you should get the following:
>    - drm fd mmap still show right content
>    - dma-buf fd mmap shows random crap that you've written into other
>      buffers
> 
> Ofc you need to make sure that an mmap actually forces the buffer into
> vram. So might need a combo of modeset+mmap, to make that happen. Plain
> mmap might just give you ptes that point into system memory, which is not
> managed by ttm like vram.

Is any move buffer good enough to trigger this, i.e. will SYSTEM -> VRAM
work too?  That'll be easier because all I need to do is map the buffer
to a crtc to force pinning to vram, then check if the mappings are
intact still ...

cheers,
  Gerd