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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat, 10 Oct 2020 13:56:00 +0200
Message-ID: <CAKMK7uEwT0TBvHtVMHeSW2gwt2nhm7mpd2pNuUHUCz=EaYM3yw@mail.gmail.com>
Subject: Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn
To:     Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
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        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Sat, Oct 10, 2020 at 1:39 PM Mauro Carvalho Chehab
<mchehab+huawei@kernel.org> wrote:
>
> Em Sat, 10 Oct 2020 12:53:49 +0200
> Daniel Vetter <daniel.vetter@ffwll.ch> escreveu:
>
> > Hi Mauro,
> >
> > You might want to read the patches more carefully, because what you're
> > demanding is what my patches do. Short summary:
> >
> > - if STRICT_FOLLOW_PFN is set:
> > a) normal memory is handled as-is (i.e. your example works) through
> > the addition of FOLL_LONGTERM. This is the "pin the pages correctly"
> > approach you're demanding
> > b) for non-page memory (zerocopy sharing before dma-buf was upstreamed
> > is the only use-case for this) it is correctly rejected with -EINVAL
> >
> > - if you do have blobby userspace which requires the zero-copy using
> > userptr to work, and doesn't have any of the fallbacks implemented
> > that you describe, this would be a regression. That's why
> > STRICT_FOLLOW_PFN can be unset. And yes there's a real security issue
> > in this usage, Marek also confirmed that the removal of the vma copy
> > code a few years ago essentially broke even the weak assumptions that
> > made the code work 10+ years ago when it was merged.
> >
> > so tdlr; Everything you described will keep working even with the new
> > flag set, and everything you demand must be implemented _is_
> > implemented in this patch series.
> >
> > Also please keep in mind that we are _not_ talking about the general
> > userptr support that was merge ~20 years ago. This patch series here
> > is _only_ about the zerocpy userptr support merged with 50ac952d2263
> > ("[media] videobuf2-dma-sg: Support io userptr operations on io
> > memory") in 2013.
>
> Ok, now it is making more sense. Please update the comments for
> patch 10/17 to describe the above.

Will do.

> We need some time to test this though, in order to check if no
> regressions were added (except the ones due to changeset 50ac952d2263).

Yeah testing of the previous patches to switch to FOLL_LONGTERM would
be really good. I also need that for habanalabs and ideally exynos
too. All the userptr for normal memory should keep working, and with
FOLL_LONGTERM it should actually work better, since with that it
should now correctly interact with pagecache and fs code, not just
with anon memory from malloc.

Thanks, Daniel

> > Why this hack was merged in 2013 when we merged dma-buf almost 2 years
> > before that I have no idea about. Imo that patch simply should never
> > have landed, and instead dma-buf support prioritized.
>
> If I recall correctly, we didn't have any DMABUF support
> at the media subsystem, back on 2013.
>
> It took some time for the DMA-BUF to arrive at media, as this
> was not a top priority. Also, there aren't many developers that
> understand the memory model well enough to implement DMA-BUF support
> and touch the VB2 code, which is quite complex, as it supports
> lots of different ways for I/O, plus works with vmalloc, DMA
> contig and DMA scatter/gather.
>
> Changes there should carefully be tested against different
> drivers, in order to avoid regressions on it.
>
> > Cheers, Daniel
>
> Thanks,
> Mauro


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch