linux-kernel - Re: [PATCH v2 09/17] mm: Add unsafe_follow

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Date:   Mon, 12 Oct 2020 15:49:07 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Kees Cook <keescook@...omium.org>,
        KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux MM <linux-mm@...ck.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn

On Mon, Oct 12, 2020 at 12:47 PM Marek Szyprowski
<m.szyprowski@...sung.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