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

From: David Rientjes <rientjes@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	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: Wed, 7 Apr 2021 09:59:55 -0700 (PDT)	[thread overview]
Message-ID: <bed5081-1f13-bc1e-6328-b2bb4517c54@google.com> (raw)
In-Reply-To: <20210406190740.4055679-1-seanjc@google.com>

On Tue, 6 Apr 2021, 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>

Always happy to see this ambiguity (SLAB_ACCOUNT vs GFP_KERNEL_ACCOUNT) 
resolved for slab allocations.

Acked-by: David Rientjes <rientjes@google.com>