Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

[Christian König]

Am 17.09.20 um 16:35 schrieb Jason Gunthorpe:
> On Thu, Sep 17, 2020 at 02:24:29PM +0200, Christian König wrote:
> > Am 17.09.20 um 14:18 schrieb Jason Gunthorpe:
> > > On Thu, Sep 17, 2020 at 02:03:48PM +0200, Christian König wrote:
> > > > Am 17.09.20 um 13:31 schrieb Jason Gunthorpe:
> > > > > On Thu, Sep 17, 2020 at 10:09:12AM +0200, Daniel Vetter wrote:
> > > > >
> > > > > > Yeah, but it doesn't work when forwarding from the drm chardev to the
> > > > > > dma-buf on the importer side, since you'd need a ton of different
> > > > > > address spaces. And you still rely on the core code picking up your
> > > > > > pgoff mangling, which feels about as risky to me as the vma file
> > > > > > pointer wrangling - if it's not consistently applied the reverse map
> > > > > > is toast and unmap_mapping_range doesn't work correctly for our needs.
> > > > > I would think the pgoff has to be translated at the same time the
> > > > > vm->vm_file is changed?
> > > > >
> > > > > The owner of the dma_buf should have one virtual address space and FD,
> > > > > all its dma bufs should be linked to it, and all pgoffs translated to
> > > > > that space.
> > > > Yeah, that is exactly like amdgpu is doing it.
> > > >
> > > > Going to document that somehow when I'm done with TTM cleanups.
> > > BTW, while people are looking at this, is there a way to go from a VMA
> > > to a dma_buf that owns it?
> > Only a driver specific one.
> Sounds OK
>
> > For TTM drivers vma->vm_private_data points to the buffer object. Not sure
> > about the drivers using GEM only.
> Why are drivers in control of the vma? I would think dma_buf should be
> the vma owner. IIRC module lifetime correctness essentially hings on
> the module owner of the struct file

Because the page fault handling is completely driver specific.

We could install some DMA-buf vmops, but that would just be another 
layer of redirection.

> > Why are you asking?
> I'm thinking about using find_vma on something that is not
> get_user_pages()'able to go to the underlying object, in this case dma
> buf.
>
> So, user VA -> find_vma -> dma_buf object -> dma_buf operations on the
> memory it represents

Ah, yes we are already doing this in amdgpu as well. But only for 
DMA-bufs or more generally buffers which are mmaped by this driver instance.

Some applications are braindead enough to mmap() a buffer and then give 
us back the CPU pointer and request to make it a handle (userptr) again.

That is clearly forbidden by OpenGL, OpenCL and Vulkan, but they use it 
anyway.

Christian.

> Jason
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