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From: Daniel Vetter <daniel@ffwll.ch>
To: "Christian König" <christian.koenig@amd.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK" 
	<linux-media@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK" 
	<linaro-mm-sig@lists.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux MM <linux-mm@kvack.org>
Subject: Re: Changing vma->vm_file in dma_buf_mmap()
Date: Wed, 16 Sep 2020 14:41:51 +0200	[thread overview]
Message-ID: <CAKMK7uGJVMj6Sb1nDTBoY8SsXc55y2ypUEsGhNLOkbbjFLuYfw@mail.gmail.com> (raw)
In-Reply-To: <b621db68-30b9-cc3f-c2c0-237a7fe4db09@amd.com>

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

  parent reply	other threads:[~2020-09-16 19:03 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-14 13:29 Changing vma->vm_file in dma_buf_mmap() Christian König
2020-09-14 13:29 ` [PATCH 1/2] drm/shmem-helpers: revert "Redirect mmap for imported dma-buf" Christian König
2020-09-15 10:39   ` Daniel Vetter
2020-09-15 11:03     ` Christian König
2020-09-15 11:07       ` Daniel Vetter
2020-09-14 13:29 ` [PATCH 2/2] mm: introduce vma_set_file function Christian König
2020-09-15  9:19   ` kernel test robot
2020-09-15 11:57   ` kernel test robot
2020-09-14 13:30 ` Changing vma->vm_file in dma_buf_mmap() Christian König
2020-09-14 14:06   ` Jason Gunthorpe
2020-09-14 18:26     ` Christian König
2020-09-16  9:53       ` Daniel Vetter
     [not found]         ` <fc8f2af7-9fc2-cb55-3065-75a4060b7c82@amd.com>
     [not found]           ` <b621db68-30b9-cc3f-c2c0-237a7fe4db09@amd.com>
2020-09-16 12:41             ` Daniel Vetter [this message]
2020-09-16 14:07         ` Jason Gunthorpe
2020-09-16 14:14           ` Christian König
2020-09-16 15:24             ` Daniel Vetter
2020-09-16 15:31               ` [Linaro-mm-sig] " Christian König
2020-09-17  6:23                 ` Daniel Vetter
2020-09-17  7:11                   ` Christian König
2020-09-17  8:09                     ` Daniel Vetter
2020-09-17 11:31                       ` Jason Gunthorpe
2020-09-17 12:03                         ` Christian König
2020-09-17 12:18                           ` Jason Gunthorpe
2020-09-17 12:24                             ` Christian König
2020-09-17 12:26                               ` Daniel Vetter
2020-09-17 14:35                               ` Jason Gunthorpe
2020-09-17 14:54                                 ` Christian König
2020-09-17 15:24                                   ` Jason Gunthorpe
2020-09-17 15:37                                     ` Daniel Vetter
2020-09-17 16:06                                       ` Christian König
2020-09-17 16:39                                         ` Jason Gunthorpe
2020-09-17 17:23                                           ` Daniel Vetter

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