linux-kernel - Re: [PATCH v2 2/3] KVM: x86/mmu: Use MMU shrinker to shrink KVM MMU memory caches

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Message-ID: <CALzav=e7utP8wT_0t2bnVjyezyde7q86F3BHTsSpR1=qVbexQg@mail.gmail.com>
Date: Mon, 28 Oct 2024 13:37:04 -0700
From: David Matlack <dmatlack@...gle.com>
To: Vipin Sharma <vipinsh@...gle.com>
Cc: Sean Christopherson <seanjc@...gle.com>, pbonzini@...hat.com, zhi.wang.linux@...il.com, 
	weijiang.yang@...el.com, mizhang@...gle.com, liangchen.linux@...il.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] KVM: x86/mmu: Use MMU shrinker to shrink KVM MMU
 memory caches

On Fri, Oct 25, 2024 at 10:37 AM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> On Thu, Oct 24, 2024 at 4:25 PM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Fri, Oct 04, 2024, Vipin Sharma wrote:
> > > +out_mmu_memory_cache_unlock:
> > > +     mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
> >
> > I've been thinking about this patch on and off for the past few weeks, and every
> > time I come back to it I can't shake the feeling that we came up with a clever
> > solution for a problem that doesn't exist.  I can't recall a single complaint
> > about KVM consuming an unreasonable amount of memory for page tables.  In fact,
> > the only time I can think of where the code in question caused problems was when
> > I unintentionally inverted the iterator and zapped the newest SPs instead of the
> > oldest SPs.
> >
> > So, I'm leaning more and more toward simply removing the shrinker integration.
>
> One thing we can agree on is that we don't need MMU shrinker in its
> current form because it is removing pages which are very well being
> used by VM instead of shrinking its cache.
>
> Regarding the current series, the biggest VM in GCE we can have 416
> vCPUs, considering each thread can have 40 pages in its cache, total
> cost gonna be around 65 MiB, doesn't seem much to me considering these
> VMs have memory in TiB. Since caches in VMs are not unbounded, I think
> it is fine to not have a MMU shrinker as its impact is miniscule in
> KVM.

I have no objection to removing the shrinker entirely.