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Date:   Wed, 31 Mar 2021 03:24:23 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] KVM: Account memory allocations for 'struct kvm_vcpu'
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References: <20210331023025.2485960-1-seanjc@google.com>
 <20210331023025.2485960-2-seanjc@google.com>
 <CANRm+CwowrYPSnFNc11j5aT2JNw_k+NOh1apoxc3raVD4RVaAg@mail.gmail.com>
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On Wed, Mar 31, 2021, Wanpeng Li wrote:
> On Wed, 31 Mar 2021 at 10:32, Sean Christopherson <seanjc@google.com> wrote:
> >
> > Use GFP_KERNEL_ACCOUNT for the vCPU allocations, the vCPUs are very much
> > tied to a single task/VM.  For x86, the allocations were accounted up
> > until the allocation code was moved to common KVM.  For all other
> > architectures, vCPU allocations were never previously accounted, but only
> > because most architectures lack accounting in general (for KVM).
> >
> > Fixes: e529ef66e6b5 ("KVM: Move vcpu alloc and init invocation to common code")
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  virt/kvm/kvm_main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 383df23514b9..3884e9f30251 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -3182,7 +3182,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
> >         if (r)
> >                 goto vcpu_decrement;
> >
> > -       vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL);
> > +       vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL_ACCOUNT);
> 
> kvm_vcpu_cache is created with SLAB_ACCOUNT flag in kvm_init(), this
> flag will guarantee further slab alloc will be charged to memcg.
> Please refer to memcg_slab_pre_alloc_hook(). So the patch is
> unnecessary.

Hmm, I missed that.  However, AFICT only SLAB/SLUB enforce SLAB_ACCOUNT, SLOB
does not appear to honor the flag.   The caveat to SLOB is that the
GFP_KERNEL_ACCOUNT will only come into play when allocating new pages, and so
allocations smaller than a page will be accounted incorrectly (I think).
But, a vcpu is larger than a page (on x86), which means the vcpu allocation will
always be correctly accounted.

I've no idea if anyone actually uses KVM+SLOB, let alone cares about accounting
in the that case.  But, it would be nice for KVM to be consistent with the other
kmem_cache usage in KVM, all of which do double up on SLAB_ACCOUNT +
GFP_KERNEL_ACCOUNT.

Maybe rewrite the changelog and drop the Fixes?