Re: [PATCH v7 3/4] KVM: x86/mmu: count KVM mmu usage in secondary pagetable stats.

[Yosry Ahmed]

On Fri, Aug 26, 2022 at 1:20 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Tue, Aug 23, 2022, Yosry Ahmed wrote:
> > Count the pages used by KVM mmu on x86 in memory stats under secondary
> > pagetable stats (e.g. "SecPageTables" in /proc/meminfo) to give better
> > visibility into the memory consumption of KVM mmu in a similar way to
> > how normal user page tables are accounted.
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> > Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c     | 16 ++++++++++++++--
> >  arch/x86/kvm/mmu/tdp_mmu.c | 12 ++++++++++++
> >  2 files changed, 26 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e418ef3ecfcb..4d38e4eba772 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1665,6 +1665,18 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, long nr)
> >       percpu_counter_add(&kvm_total_used_mmu_pages, nr);
> >  }
> >
> > +static void kvm_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> > +{
> > +     kvm_mod_used_mmu_pages(kvm, +1);
> > +     kvm_account_pgtable_pages((void *)sp->spt, +1);
> > +}
> > +
> > +static void kvm_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> > +{
> > +     kvm_mod_used_mmu_pages(kvm, -1);
> > +     kvm_account_pgtable_pages((void *)sp->spt, -1);
> > +}
>
> Hrm, this is causing build on x86 issues for me.  AFAICT, modpost doesn't detect
> that this creates a new module dependency on __mod_lruvec_page_state() and so doesn't
> refresh vmlinux.symvers.
>
>   ERROR: modpost: "__mod_lruvec_page_state" [arch/x86/kvm/kvm.ko] undefined!
>   make[2]: *** [scripts/Makefile.modpost:128: modules-only.symvers] Error 1
>   make[1]: *** [Makefile:1769: modules] Error 2
>   make[1]: *** Waiting for unfinished jobs....
>   Kernel: arch/x86/boot/bzImage is ready  (#128)
>   make[1]: Leaving directory '/usr/local/google/home/seanjc/build/kernel/vm'
>   make: *** [Makefile:222: __sub-make] Error 2
>
> Both gcc and clang yield the same behavior, so I doubt it's the compiler doing
> something odd.  Cleaning the build makes the problem go away, but that's a poor
> band-aid.
>
> If I squash this with the prior patch that adds kvm_account_pgtable_pages() to
> kvm_host.h, modpost detects the need to refresh and all is well.
>
> Given that ARM doesn't support building KVM as a module, i.e. can't run afoul
> of whatever modpost weirdness I'm hitting, I'm inclined to squash this with the
> previous patch and punt on the modpost issue so that we can get this merged.
>
> Any objections?  Or thoughts on what's going wrong?

I am not familiar at all with modpost so I have no idea what's going
wrong, but I have no problem with squashing the patches if you think
this works best.