linux-kernel - Re: [PATCH v2 4/6] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages

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Date:   Wed, 27 Jul 2022 10:41:39 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        "Mingwei Zhang" <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 4/6] KVM: x86/mmu: Track the number of TDP MMU pages,
 but not the actual pages

On Sat, Jul 23, 2022 at 01:23:23AM +0000, Sean Christopherson wrote:

<snip>

> @@ -386,16 +385,18 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn,
>  static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
>  			      bool shared)
>  {
> +	atomic64_dec(&kvm->arch.tdp_mmu_pages);
> +
> +	if (!sp->nx_huge_page_disallowed)
> +		return;
> +
Does this read of sp->nx_huge_page_disallowed also need to be protected by
tdp_mmu_pages_lock in shared path?

Thanks
Yan

>  	if (shared)
>  		spin_lock(&kvm->arch.tdp_mmu_pages_lock);
>  	else
>  		lockdep_assert_held_write(&kvm->mmu_lock);
>  
> -	list_del(&sp->link);
> -	if (sp->nx_huge_page_disallowed) {
> -		sp->nx_huge_page_disallowed = false;
> -		untrack_possible_nx_huge_page(kvm, sp);
> -	}
> +	sp->nx_huge_page_disallowed = false;
> +	untrack_possible_nx_huge_page(kvm, sp);
>  
>  	if (shared)
>  		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> @@ -1132,9 +1133,7 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
>  		tdp_mmu_set_spte(kvm, iter, spte);
>  	}
>  
> -	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> -	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
> -	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> +	atomic64_inc(&kvm->arch.tdp_mmu_pages);
>  
>  	return 0;
>  }
> -- 
> 2.37.1.359.gd136c6c3e2-goog
>