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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id v9-20020a17090a520900b00218e8143380sm1955302pjh.28.2022.12.14.16.11.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Dec 2022 16:11:19 -0800 (PST) Date: Thu, 15 Dec 2022 00:11:15 +0000 From: Sean Christopherson To: Robert Hoo Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Greg Thelen , David Matlack , Ben Gardon , Mingwei Zhang Subject: Re: [PATCH 3/5] KVM: x86/mmu: Re-check under lock that TDP MMU SP hugepage is disallowed Message-ID: References: <20221213033030.83345-1-seanjc@google.com> <20221213033030.83345-4-seanjc@google.com> <247fcfc6de8ec08d0667de125e707046dce903fc.camel@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <247fcfc6de8ec08d0667de125e707046dce903fc.camel@linux.intel.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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. 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. >