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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon, 12 Oct 2020 15:49:07 +0200
Message-ID: <CAKMK7uGLOubJrr9A=usxkoVx+nXDW3bxhykfeU-TYXTUZyBoeQ@mail.gmail.com>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn
To:     Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        Jan Kara <jack@suse.cz>, Kees Cook <keescook@chromium.org>,
        KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux MM <linux-mm@kvack.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>
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On Mon, Oct 12, 2020 at 12:47 PM Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
>
> Hi Jason,
>
> On 09.10.2020 14:48, Jason Gunthorpe wrote:
> > On Fri, Oct 09, 2020 at 02:37:23PM +0200, Mauro Carvalho Chehab wrote:
> >
> >> I'm not a mm/ expert, but, from what I understood from Daniel's patch
> >> description is that this is unsafe *only if*  __GFP_MOVABLE is used.
> > No, it is unconditionally unsafe. The CMA movable mappings are
> > specific VMAs that will have bad issues here, but there are other
> > types too.
>
> I'm trying to follow this thread, but I really wonder what do you mean
> by CMA movable mappings? If a buffer has been allocated from CMA and
> used for DMA, it won't be moved in the memory. It will stay at the same
> physical memory address all the time until freed by the owner. It just a
> matter of proper usage count tracking to delay freeing if it is still
> used somewhere.

 Yup. The problem is that this usage count tracking doesn't exist. And
drivers could at least in theory treat CMA like vram and swap buffers
in&out of it, so just refcounting the userspace vma isn't enough. In
practice, right now, it might be enough for CMA drivers though (but
there's more that's possible here).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch