Messages in this thread | | | Date | Fri, 28 Jul 2023 23:02:46 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout |
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On 28.07.23 22:50, Linus Torvalds wrote: > On Fri, 28 Jul 2023 at 13:33, David Hildenbrand <david@redhat.com> wrote: >> >> So would you rather favor a FOLL_NUMA that has to be passed from the >> outside by selected callers or a FOLL_NUMA that is set on the GUP path >> unconditionally (but left clear for follow_page())? > > I'd rather see the FOLL_NUMA that has to be set by odd cases, and that > is never set by any sane user.
Thanks!
> > And it should not be called FOLL_NUMA. It should be called something > else. Because *not* having it doesn't disable following pages across > NUMA boundaries, and the name is actively misleading. > > It sounds like what KVM actually wants is a "Do NOT follow NUMA pages, > I'll force a page fault". > > And the fact that KVM wants a fault for NUMA pages shouldn't mean that > others - who clearly cannot care - get that insane behavior by > default.
For KVM it represents actual CPU access. To map these pages into the VM MMU we have to look them up from the process -- in the context of the faulting CPU. So it makes a lot of sense for KVM. (which is also where autonuma gets heavily used)
> > The name should reflect that, instead of being the misleading mess of > FOLL_FORCE and bad naming that it is now. > > So maybe it can be called "FOLL_HONOR_NUMA_FAULT" or something, to > make it clear that it's the *opposite* of FOLL_FORCE, and that it > honors the NUMA faulting that nobody should care about.
Naming sounds much better to me.
> > Then the KVM code can have a big comment about *why* it sets that bit.
Yes.
> > Hmm? Can we please aim for something that is understandable and > documented? No odd implicit rules. No "force NUMA fault even when it > makes no sense". No tie-in with FOLL_FORCE.
I mean, I messed all that FOLL_NUMA handling up because I was very confused. So I'm all for better documentation.
Can we get a simple revert in first (without that FOLL_FORCE special casing and ideally with a better name) to handle stable backports, and I'll follow-up with more documentation and letting GUP callers pass in that flag instead?
That would help a lot. Then we also have more time to let that "move it to GUP callers" mature a bit in -next, to see if we find any surprises?
-- Cheers,
David / dhildenb
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