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

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Date:   Wed, 17 Mar 2021 11:55:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
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 17/03/21 11:53, Marc Zyngier wrote:
> 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.

Ah, these are the EL2 pages, not the stage2 page tables, right?  If so, 
sorry for the noise.

Paolo