Changing vma->vm_file in dma_buf_mmap()

[Daniel Vetter]

On Wed, Sep 16, 2020 at 1:45 PM Christian König
<christian.koenig at amd.com> wrote:
>
> [SNIP]
>
> But Jason pointed me to the right piece of code. See this comment in in mmap_region():
>
> /* ->mmap() can change vma->vm_file, but must guarantee that
> * vma_link() below can deny write-access if VM_DENYWRITE is set
> * and map writably if VM_SHARED is set. This usually means the
> * new file must not have been exposed to user-space, yet.
> */
> vma->vm_file = get_file(file);
> error = call_mmap(file, vma);
>
>
> So changing vma->vm_file is allowed at least under certain circumstances.
>
> Only the "file must not have been exposed to user-space, yet" part still needs double checking. Currently working on that.
>
>
> Ok, I think we can guarantee for all DMA-bufs what is required here.
>
> While searching the code I've found that at least vgem_prime_mmap() and i915_gem_dmabuf_mmap() are doing the same thing of modifying vma->vm_file.
>
> So I'm leaning towards that this works as expected and we should just document this properly.
>
> Daniel and Jason what do you think?

Well I can claim I started this, so I started out with naively
assuming that it Just Works :-)

I think we already have vgem/i915 prime testcases in igt which try to
excercise this mmap forwarding, including provoking pte shoot-downs.
So I think best would be if you could also add a variant for amdgpu,
to make sure this really works and keeps working.

Plus ofc the documentation patch so that core mm folks can officially
ack this as legit.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch