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

[Rob Herring]

On Wed, Nov 13, 2019 at 2:18 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>
> On Wed, Nov 13, 2019 at 8:39 AM Gerd Hoffmann <kraxel at redhat.com> wrote:
> >
> >   Hi,
> >
> > > > > >> VRAM helpers use drm_gem_ttm_mmap(), which wraps ttm_bo_mmap_obj().
> > > > > >> These changes should be transparent.
> > > > > >
> > > > > > There's still the issue that for dma-buf mmap vs drm mmap you use
> > > > > > different f_mapping, which means ttm's pte shootdown won't work correctly
> > > > > > for dma-buf mmaps. But yeah normal operation for ttm/vram helpers should
> > > > > > be fine.
> > > > >
> > > > > VRAM helpers don't support dmabufs.
> > > >
> > > > It's not that simple.  Even when not supporting dma-buf export and
> > > > import it is still possible to create dma-bufs, import them into the
> > > > same driver (which doesn't actually export+import but just grabs a gem
> > > > object reference instead) and also to mmap them via prime/dma-buf code
> > > > path ...
> >
> > ... but after looking again I think we are all green here.  Given that
> > only self-import works we'll only see vram gem objects in the mmap code
> > path, which should have everything set up correctly.  Same goes for qxl.
> >
> > All other ttm drivers still use the old mmap code path, so all green
> > there too I think.  Also I somehow doubt dma-buf mmap vs. drm mmap ends
> > up using different f_mapping, ttm code has a WARN_ON in ttm_bo_vm_open()
> > which would fire should that be the case.
> >
> > Do imported dma-bufs hit the drm mmap code path in the first place?
> > Wouldn't mmap be handled by the exporting driver?
>
> drm_gem_dmabuf_mmap -> obj->funcs->mmap -> ttm_bo_mmap_obj
>
> And I'm not seeing anyone adjusting vm_file->f_mapping anywhere here at all.
>
> Note to hit this you need userspace to
> - handle2fd on a buffer to create a dma-buf fd
> - call mmap directly on that dma-buf fd

That was exactly the vgem IGT test that prompted $subject fix. (With
vgem modified to use shmem helpers)

Rob