Re: [PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

From: Ben Gardon <bgardon@google.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: linux-arch@vger.kernel.org, Junaid Shahid <junaids@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	kvm@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Marc Zyngier <maz@kernel.org>, Joerg Roedel <joro@8bytes.org>,
	Peter Shier <pshier@google.com>,
	linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Peter Feiner <pfeiner@google.com>,
	kvmarm@lists.cs.columbia.edu, Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty
Date: Wed, 24 Jun 2020 11:03:32 -0700	[thread overview]
Message-ID: <CANgfPd8gYX1Fm1vEcfnEBXn_MjRxLHdgQAS=TAHQiOMNMrhFGA@mail.gmail.com> (raw)
In-Reply-To: <20200622200822.4426-6-sean.j.christopherson@intel.com>

On Mon, Jun 22, 2020 at 1:09 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> Attempt to allocate a new object instead of crashing KVM (and likely the
> kernel) if a memory cache is unexpectedly empty.  Use GFP_ATOMIC for the
> allocation as the caches are used while holding mmu_lock.  The immediate
> BUG_ON() makes the code unnecessarily explosive and led to confusing
> minimums being used in the past, e.g. allocating 4 objects where 1 would
> suffice.
>
Reviewed-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index ba70de24a5b0..5e773564ab20 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1060,6 +1060,15 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
>         local_irq_enable();
>  }
>
> +static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
> +                                              gfp_t gfp_flags)
> +{
> +       if (mc->kmem_cache)
> +               return kmem_cache_zalloc(mc->kmem_cache, gfp_flags);
> +       else
> +               return (void *)__get_free_page(gfp_flags);
> +}
> +
>  static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
>  {
>         void *obj;
> @@ -1067,10 +1076,7 @@ static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
>         if (mc->nobjs >= min)
>                 return 0;
>         while (mc->nobjs < ARRAY_SIZE(mc->objects)) {
> -               if (mc->kmem_cache)
> -                       obj = kmem_cache_zalloc(mc->kmem_cache, GFP_KERNEL_ACCOUNT);
> -               else
> -                       obj = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
> +               obj = mmu_memory_cache_alloc_obj(mc, GFP_KERNEL_ACCOUNT);
>                 if (!obj)
>                         return mc->nobjs >= min ? 0 : -ENOMEM;
>                 mc->objects[mc->nobjs++] = obj;
> @@ -1118,8 +1124,11 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
>  {
>         void *p;
>
> -       BUG_ON(!mc->nobjs);
> -       p = mc->objects[--mc->nobjs];
> +       if (WARN_ON(!mc->nobjs))
> +               p = mmu_memory_cache_alloc_obj(mc, GFP_ATOMIC | __GFP_ACCOUNT);
> +       else
> +               p = mc->objects[--mc->nobjs];
> +       BUG_ON(!p);
>         return p;
>  }
>
> --
> 2.26.0
>
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm