linux-kernel - Re: [PATCH 09/12] KVM: MMU: coalesce zapping page after mmu_sync

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Date:	Thu, 25 Feb 2016 11:15:45 +0900
From:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	guangrong.xiao@...ux.intel.com, mtosatti@...hat.com
Subject: Re: [PATCH 09/12] KVM: MMU: coalesce zapping page after
 mmu_sync_children

On 2016/02/24 22:17, Paolo Bonzini wrote:
> Move the call to kvm_mmu_flush_or_zap outside the loop.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/mmu.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 725316df32ec..6d47b5c43246 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2029,24 +2029,27 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
>   	struct mmu_page_path parents;
>   	struct kvm_mmu_pages pages;
>   	LIST_HEAD(invalid_list);
> +	bool flush = false;
>   
>   	while (mmu_unsync_walk(parent, &pages)) {
>   		bool protected = false;
> -		bool flush = false;
>   
>   		for_each_sp(pages, sp, parents, i)
>   			protected |= rmap_write_protect(vcpu, sp->gfn);
>   
> -		if (protected)
> +		if (protected) {
>   			kvm_flush_remote_tlbs(vcpu->kvm);
> +			flush = false;
> +		}
>   
>   		for_each_sp(pages, sp, parents, i) {
>   			flush |= kvm_sync_page(vcpu, sp, &invalid_list);
>   			mmu_pages_clear_parents(&parents);
>   		}
> -		kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
>   		cond_resched_lock(&vcpu->kvm->mmu_lock);

This may release the mmu_lock before committing the zapping.
Is it safe?  If so, we may want to see the reason in the changelog.

  Takuya

>   	}
> +
> +	kvm_mmu_flush_or_zap(vcpu, &invalid_list, false, flush);
>   }
>   
>   static void __clear_sp_write_flooding_count(struct kvm_mmu_page *sp)
>