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

Christian König
2020-09-17T14:59:55Z

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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