linux-kernel - Re: [PATCH] KVM: arm: memcg awareness

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Message-ID: <87im5qhwzx.wl-maz@kernel.org>
Date:   Wed, 17 Mar 2021 10:53:38 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: [PATCH] KVM: arm: memcg awareness

On Wed, 17 Mar 2021 10:40:23 +0000,
Paolo Bonzini <pbonzini@...hat.com> 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.

	M.

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