linux-kernel - Re: Changing vma->vm_file in dma_buf

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Message-ID: <CAKMK7uGJVMj6Sb1nDTBoY8SsXc55y2ypUEsGhNLOkbbjFLuYfw@mail.gmail.com>
Date:   Wed, 16 Sep 2020 14:41:51 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: Changing vma->vm_file in dma_buf_mmap()

On Wed, Sep 16, 2020 at 1:45 PM Christian König
<christian.koenig@....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