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

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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





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