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

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Date:   Sat, 31 Oct 2020 22:22:11 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
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 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.

  thanks,
-- 
John Hubbard
NVIDIA