linux-kernel - Re: [PATCH 3/5] KVM: x86/mmu: Re-check under lock that TDP MMU SP hugepage is disallowed

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Date:   Thu, 15 Dec 2022 14:26:05 +0800
From:   Robert Hoo <robert.hu@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Greg Thelen <gthelen@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH 3/5] KVM: x86/mmu: Re-check under lock that TDP MMU SP
 hugepage is disallowed

On Thu, 2022-12-15 at 00:11 +0000, Sean Christopherson wrote:
> On Wed, Dec 14, 2022, Robert Hoo wrote:
> > On Tue, 2022-12-13 at 03:30 +0000, Sean Christopherson wrote:
> > > ---
> > >  arch/x86/kvm/mmu/tdp_mmu.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c
> > > b/arch/x86/kvm/mmu/tdp_mmu.c
> > > index e2e197d41780..fd4ae99790d7 100644
> > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > > @@ -1203,7 +1203,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu,
> > > struct kvm_page_fault *fault)
> > >  		if (fault->huge_page_disallowed &&
> > >  		    fault->req_level >= iter.level) {
> > >  			spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> > > -			track_possible_nx_huge_page(kvm, sp);
> > > +			if (sp->nx_huge_page_disallowed)
> > > +				track_possible_nx_huge_page(kvm, sp);
> > >  			spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> > >  		}
> > >  	}
> > 
> > Is this possible?
> > The aforementioned situation happened, i.e. before above hunk
> > track_possible_nx_huge_page(), the sp is zapped by some other task,
> > tdp_mmu_unlink_sp() --> untrack_possible_nx_huge_page(kvm, sp):
> 
> It's possible for untrack_possible_nx_huge_page() to be called before
> the above
> snippet, but the stat won't be decremented in that case since the
> page won't be on
> the list of possible NX huge pages, i.e. list_empty() will be true.

Right, I was fooled by the name of list_empty(), it's actually
list_node_empty(). Thanks for explaining.
> 
>   void untrack_possible_nx_huge_page(struct kvm *kvm, struct
> kvm_mmu_page *sp)
>   {
> 	if (list_empty(&sp->possible_nx_huge_page_link))
> 		return;
> 
> 	--kvm->stat.nx_lpage_splits;
> 
> And by not calling track_possible_nx_huge_page() (this bug fix),
> nx_lpage_splits
> won't be incorrectly incremented.
> 
> > 
> > --kvm->stat.nx_lpage_splits;
> > 
> > But looks like the stat for this sp hasn't been increased yet.
> >