linux-kernel - Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL

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Date:   Sun, 1 Nov 2020 11:30:49 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

On Sun, Nov 1, 2020 at 6:22 AM John Hubbard <jhubbard@...dia.com> wrote:
>
> On 10/31/20 7:45 AM, Daniel Vetter wrote:
> > On Sat, Oct 31, 2020 at 3:55 AM John Hubbard <jhubbard@...dia.com> wrote:
> >> On 10/30/20 3:08 AM, Daniel Vetter wrote:
> ...
> >> By removing this check from this location, and changing from
> >> pin_user_pages_locked() to pin_user_pages_fast(), I *think* we end up
> >> losing the check entirely. Is that intended? If so it could use a comment
> >> somewhere to explain why.
> >
> > Yeah this wasn't intentional. I think I needed to drop the _locked
> > version to prep for FOLL_LONGTERM, and figured _fast is always better.
> > But I didn't realize that _fast doesn't have the vma checks, gup.c got
> > me a bit confused.
>
> Actually, I thought that the change to _fast was a very nice touch, btw.
>
> >
> > I'll remedy this in all the patches where this applies (because a
> > VM_IO | VM_PFNMAP can point at struct page backed memory, and that
> > exact use-case is what we want to stop with the unsafe_follow_pfn work
> > since it wreaks things like cma or security).
> >
> > Aside: I do wonder whether the lack for that check isn't a problem.
> > VM_IO | VM_PFNMAP generally means driver managed, which means the
> > driver isn't going to consult the page pin count or anything like that
> > (at least not necessarily) when revoking or moving that memory, since
> > we're assuming it's totally under driver control. So if pup_fast can
> > get into such a mapping, we might have a problem.
> > -Daniel
> >
>
> Yes. I don't know why that check is missing from the _fast path.
> Probably just an oversight, seeing as how it's in the slow path. Maybe
> the appropriate response here is to add a separate patch that adds the
> check.
>
> I wonder if I'm overlooking something, but it certainly seems correct to
> do that.

You'll need the mmap_sem to get at the vma to be able to do this
check. If you add that to _fast, you made it as fast as the slow one.
Plus there's _fast_only due to locking recurion issues in fast-paths
(I assume, I didn't check all the callers).

I'm just wondering whether we have a bug somewhere with device
drivers. For CMA regions we always check in try_grab_page, but for dax
I'm not seeing where the checks in the _fast fastpaths are, and that
all still leaves random device driver mappings behind which aren't
backed by CMA but still point to something with a struct page behind
it. I'm probably just missing something, but no idea what.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch