Re: [PATCH v6 6/8] KVM: Handle page fault for private memory

[Kirill A. Shutemov]

On Fri, Jun 17, 2022 at 09:30:53PM +0000, Sean Christopherson wrote:
> > @@ -4088,7 +4144,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, 
> > struct kvm_page_fault *fault
> >             read_unlock(&vcpu->kvm->mmu_lock);
> >     else
> >             write_unlock(&vcpu->kvm->mmu_lock);
> > -   kvm_release_pfn_clean(fault->pfn);
> > +
> > +   if (fault->is_private)
> > +           kvm_private_mem_put_pfn(fault->slot, fault->pfn);
> 
> Why does the shmem path lock the page, and then unlock it here?

Lock is require to avoid race with truncate / punch hole. Like if truncate
happens after get_pfn(), but before it gets into SEPT we are screwed.

> Same question for why this path marks it dirty?  The guest has the page mapped
> so the dirty flag is immediately stale.

If page is clean and refcount is not elevated, vmscan is free to drop the
page from page cache. I don't think we want this.

> In other words, why does KVM need to do something different for private pfns?

Because in the traditional KVM memslot scheme, core mm takes care about
this.

The changes in v7 is wrong. Page has be locked until it lends into SEPT and
must make it dirty before unlocking.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov