linux-kernel - Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow

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Date:   Wed, 19 Jul 2023 14:09:01 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     David Stevens <stevensd@...omium.org>
CC:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Xu <peterx@...hat.com>,
        <linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to
 __kvm_follow_pfn

On Tue, Jul 04, 2023 at 04:50:50PM +0900, David Stevens wrote:
> @@ -4451,7 +4461,8 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  
>  out_unlock:
>  	write_unlock(&vcpu->kvm->mmu_lock);
> -	kvm_release_pfn_clean(fault->pfn);
> +	if (fault->is_refcounted_page)
> +		kvm_set_page_accessed(pfn_to_page(fault->pfn));
For a refcounted page, as now KVM puts its ref early in kvm_faultin_pfn(),
should this kvm_set_page_accessed() be placed before unlocking mmu_lock?

Otherwise, if the user unmaps a region (which triggers kvm_unmap_gfn_range()
with mmu_lock holding for write), and release the page, and if the two
steps happen after checking page_count() in kvm_set_page_accessed() and
before mark_page_accessed(), the latter function may mark accessed to a page
that is released or does not belong to current process.

Is it true?

>  	return r;
>  }
>  
> @@ -4529,7 +4540,8 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
>  
>  out_unlock:
>  	read_unlock(&vcpu->kvm->mmu_lock);
> -	kvm_release_pfn_clean(fault->pfn);
> +	if (fault->is_refcounted_page)
> +		kvm_set_page_accessed(pfn_to_page(fault->pfn));
>  	return r;
>  }
Ditto.