linux-kernel - Re: [PATCH 10/12] KVM: MMU: allow locklessly access shadow page table out of vcpu thread

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Date:	Wed, 07 Aug 2013 21:19:53 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
CC:	gleb@...hat.com, avi.kivity@...il.com, mtosatti@...hat.com,
	pbonzini@...hat.com, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH 10/12] KVM: MMU: allow locklessly access shadow page table
 out of vcpu thread

On 08/07/2013 09:09 PM, Takuya Yoshikawa wrote:
> On Tue, 30 Jul 2013 21:02:08 +0800
> Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com> wrote:
> 
>> @@ -2342,6 +2358,13 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>>  	 */
>>  	kvm_flush_remote_tlbs(kvm);
>>  
>> +	if (kvm->arch.rcu_free_shadow_page) {
>> +		sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
>> +		list_del_init(invalid_list);
>> +		call_rcu(&sp->rcu, free_pages_rcu);
>> +		return;
>> +	}
>> +
>>  	list_for_each_entry_safe(sp, nsp, invalid_list, link) {
>>  		WARN_ON(!sp->role.invalid || sp->root_count);
>>  		kvm_mmu_free_page(sp);
> 
> Shouldn't we avoid calling call_rcu() when we are holding mmu_lock?

Using call_rcu() to free pages is a rare case that happen only between
lockless write-protection and zapping shadow pages, so i think we do
not need to care this case too much.

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