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    SubjectRe: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM
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    On 10/31/20 7:45 AM, Daniel Vetter wrote:
    > On Sat, Oct 31, 2020 at 3:55 AM John Hubbard <jhubbard@nvidia.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

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