Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

[John Hubbard]

On 10/31/20 7:45 AM, Daniel Vetter wrote:
> On Sat, Oct 31, 2020 at 3:55 AM John Hubbard <jhubbard@xxxxxxxxxx> 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