Re: [PATCH] KVM: arm: memcg awareness

From: Marc Zyngier
Date: Wed Mar 17 2021 - 07:04:18 EST


On Wed, 17 Mar 2021 10:55:00 +0000,
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 17/03/21 11:53, Marc Zyngier wrote:
> > On Wed, 17 Mar 2021 10:40:23 +0000,
> > Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> >>
> >> On 17/03/21 10:10, Marc Zyngier wrote:
> >>>> @@ -366,7 +366,7 @@ static int hyp_map_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> >>>> if (WARN_ON(level == KVM_PGTABLE_MAX_LEVELS - 1))
> >>>> return -EINVAL;
> >>>> - childp = (kvm_pte_t *)get_zeroed_page(GFP_KERNEL);
> >>>> + childp = (kvm_pte_t *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> >>> No, this is wrong.
> >>>
> >>> You cannot account the hypervisor page tables to the guest because we
> >>> don't ever unmap them, and that we can't distinguish two data
> >>> structures from two different VMs occupying the same page.
> >>
> >> If you never unmap them, there should at least be a shrinker to get
> >> rid of unused pages in the event of memory pressure.
> >
> > We don't track where these pages are coming from or whether they can
> > safely be unmapped. Until we can track such ownership and deal with
> > page sharing, these mappings have to stay,
> >
> > At most, this represent the amount of memory required to map the whole
> > of the linear mapping.
>
> Ah, these are the EL2 pages, not the stage2 page tables, right? If
> so, sorry for the noise.

Yes, EL2 page tables when running non-VHE. VHE doesn't have that
problem for obvious reasons. Stage-2 page tables can be completely
discarded at any point, and the MMU notifiers already deal with that.

M.

--
Without deviation from the norm, progress is not possible.