Re: [PATCH v2] KVM: Explicitly use GFP_KERNEL_ACCOUNT for 'struct kvm_vcpu' allocations

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Wanpeng Li <kernellwp@gmail.com>
Subject: Re: [PATCH v2] KVM: Explicitly use GFP_KERNEL_ACCOUNT for 'struct kvm_vcpu' allocations
Date: Thu, 8 Apr 2021 14:00:12 +0200	[thread overview]
Message-ID: <bf29a3f3-cb55-dbfd-36da-708cc67d1d1a@redhat.com> (raw)
In-Reply-To: <20210406190740.4055679-1-seanjc@google.com>

On 06/04/21 21:07, Sean Christopherson wrote:
> Use GFP_KERNEL_ACCOUNT when allocating vCPUs to make it more obvious that
> that the allocations are accounted, to make it easier to audit KVM's
> allocations in the future, and to be consistent with other cache usage in
> KVM.
> 
> When using SLAB/SLUB, this is a nop as the cache itself is created with
> SLAB_ACCOUNT.
> 
> When using SLOB, there are caveats within caveats.  SLOB doesn't honor
> SLAB_ACCOUNT, so passing GFP_KERNEL_ACCOUNT will result in vCPU
> allocations now being accounted.   But, even that depends on internal
> SLOB details as SLOB will only go to the page allocator when its cache is
> depleted.  That just happens to be extremely likely for vCPUs because the
> size of kvm_vcpu is larger than the a page for almost all combinations of
> architecture and page size.  Whether or not the SLOB behavior is by
> design is unknown; it's just as likely that no SLOB users care about
> accounding and so no one has bothered to implemented support in SLOB.
> Regardless, accounting vCPU allocations will not break SLOB+KVM+cgroup
> users, if any exist.
> 
> Cc: Wanpeng Li <kernellwp@gmail.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> 
> v2: Drop the Fixes tag and rewrite the changelog since this is a nop when
>      using SLUB or SLAB. [Wanpeng]
> 
>   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 0a481e7780f0..580f98386b42 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3192,7 +3192,7 @@ 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);
>   	if (!vcpu) {
>   		r = -ENOMEM;
>   		goto vcpu_decrement;
> 

Queued, thanks.

Paolo