Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout

[David Hildenbrand]

On 28.07.23 21:50, Peter Xu wrote:
> On Fri, Jul 28, 2023 at 09:40:54PM +0200, David Hildenbrand wrote:
> > Hmm. So three alternatives I see:
> >
> > 1) Use FOLL_FORCE in follow_page() to unconditionally disable protnone
> >     checks. Alternatively, have an internal FOLL_NO_PROTNONE flag if we
> >     don't like that.
> >
> > 2) Revert the commit and reintroduce unconditional FOLL_NUMA without
> >     FOLL_FORCE.
> >
> > 3) Have a FOLL_NUMA that callers like KVM can pass.
>
> I'm afraid 3) means changing numa balancing to opt-in, probably no-go for
> any non-kvm gup users as that could start to break there, even if making
> smaps/follow_page happy again.
>
> I keep worrying 1) on FOLL_FORCE abuse.
>
> So I keep my vote on 2).

Linus had a point that we can nowadays always set FOLL_NUMA, even with 
FOLL_FORCE.

... so maybe we want to let GUP always set FOLL_NUMA (even with 
FOLL_FORCE) and follow_page() never set FOLL_NUMA.

That would at least decouple FOLL_NUMA from FOLL_FORCE.

--
Cheers,

David / dhildenb