linux-kernel - Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf

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Message-ID: <20200917143551.GG8409@ziepe.ca>
Date:   Thu, 17 Sep 2020 11:35:51 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     christian.koenig@....com
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Linux MM <linux-mm@...ck.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] Changing vma->vm_file in dma_buf_mmap()

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

> 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

Jason