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

From: "Christian König" <ckoenig.leichtzumerken@gmail.com>
To: Jason Gunthorpe <jgg@ziepe.ca>, christian.koenig@amd.com
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK" 
	<linaro-mm-sig@lists.linaro.org>, Linux MM <linux-mm@kvack.org>,
	Daniel Vetter <daniel@ffwll.ch>,
	Andrew Morton <akpm@linux-foundation.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK" 
	<linux-media@vger.kernel.org>
Subject: Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()
Date: Thu, 17 Sep 2020 16:54:44 +0200	[thread overview]
Message-ID: <5b330920-c789-fac7-e9b1-49f3bc1097a8@gmail.com> (raw)
In-Reply-To: <20200917143551.GG8409@ziepe.ca>

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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