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

Jason Gunthorpe
2020-09-17T12:19:47Z

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?

Jason