Messages in this thread | | | Date | Fri, 28 Jul 2023 21:52:11 +0200 | Subject | Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout | From | David Hildenbrand <> |
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On 28.07.23 21:39, Peter Xu wrote: > Hi, Linus, > > On Fri, Jul 28, 2023 at 09:18:45AM -0700, Linus Torvalds wrote: >> The original reason for FOLL_NUMA simply does not exist any more. We >> know exactly when a page is marked for NUMA faulting, and we should >> simply *ignore* it for GUP and follow_page(). >> >> I think we should treat a NUMA-faulting page as just being present >> (and not NUMA-fault it). > > But then does it means that any gup-only user will have numa balancing > completely disabled? Since as long as the page will only be accessed by > GUP, the numa balancing will never trigger anyway.. I think KVM is > manipulating guest pages just like that. Not sure whether it means it'll > void the whole numa effort there. > > If we allow that GUP from happening (taking protnone as present) I assume > it'll also stop any further numa balancing on this very page to trigger > too, because even if some page fault handler triggered on this protnone > page later that is not GUP anymore, when it wants to migrate the page to > the other numa node it'll see someone is holding a reference on it already, > and then we should give up the balancing. > > So to me FOLL_NUMA (or any identifier like it.. just to describe the > caller's need; some may really just want to fetch the pfn/page) still makes > sense. But maybe I totally misunderstood above..
Yes, I agree, took me a bit longer to realize (being a KVM developer :) ... I'm really ready for the weekend).
So if this series is not acceptable then better revert that commit -- or let callers like KVM specify FOLL_NUMA.
-- Cheers,
David / dhildenb
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