Re: [PATCH 1/2] KVM: Account memory allocations for 'struct kvm_vcpu'

[Wanpeng Li]

On Wed, 31 Mar 2021 at 11:24, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Mar 31, 2021, Wanpeng Li wrote:
> > On Wed, 31 Mar 2021 at 10:32, Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >  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

I asked maintainer Christoph in 2013, he told me "Well, I have never
seen non experimental systems that use SLOB. Others have claimed they
exist. It's mostly of academic interest."

> 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?

Agreed.

    Wanpeng