Re: [PATCH 09/12] KVM: MMU: coalesce zapping page after mmu_sync_children

[Xiao Guangrong]

On 02/25/2016 10:15 AM, Takuya Yoshikawa wrote:
> 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@xxxxxxxxxx>
> > ---
> >    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.

It is unsafe indeed, please do not do it.