linux-kernel - Re: [PATCH v2] KVM: Explicitly use GFP_KERNEL_ACCOUNT for 'struct kvm

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Date:   Wed, 7 Apr 2021 17:03:17 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: Explicitly use GFP_KERNEL_ACCOUNT for 'struct
 kvm_vcpu' allocations

On Wed, 7 Apr 2021 at 03:07, Sean Christopherson <seanjc@...gle.com> 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@...il.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Wanpeng Li <wanpengli@...cent.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;
> --
> 2.31.0.208.g409f899ff0-goog
>