linux-kernel - Re: [Patch v4 03/18] KVM: x86/mmu: Track count of pages in KVM MMU page caches globally

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Date:   Thu, 9 Mar 2023 16:36:26 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, bgardon@...gle.com,
        jmattson@...gle.com, mizhang@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Patch v4 03/18] KVM: x86/mmu: Track count of pages in KVM MMU
 page caches globally

On Thu, Mar 9, 2023 at 4:22 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Mon, Mar 06, 2023 at 02:41:12PM -0800, Vipin Sharma wrote:
> >
> >  static void mmu_free_memory_caches(struct kvm_vcpu *vcpu)
> >  {
> >       kvm_mmu_free_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache);
> > -     kvm_mmu_free_memory_cache(&vcpu->arch.mmu_shadow_page_cache);
> > +     mutex_lock(&vcpu->arch.mmu_shadow_page_cache_lock);
> > +     mmu_free_sp_memory_cache(&vcpu->arch.mmu_shadow_page_cache);
> > +     mutex_unlock(&vcpu->arch.mmu_shadow_page_cache_lock);
>
> Is this lock necessary (even when the shrinker is hooked up)?
> mmu_free_memory_caches() is only called when KVM fails to create a vCPU
> (before it has been added to vcpu_array) or during VM destruction (after
> the VM has been removed from vm_list).

My approach was if shrinker ran just before VM destruction and removed
pages, it would reduce nobjs variable in the cache. Now, when the VM
is being destroyed, mmu_free_sp_memory_cache() will first read the
nobjs variable to update the global counter and free the cache. To be
sure that the latest value is read and there is no memory ordering
issue I used mutex.

I discussed with Sean offline and he pointed out that x86 is strongly
ordered and mutex is not needed when freeing memory caches.