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

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Date:   Wed, 24 Jun 2020 11:03:32 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Marc Zyngier <maz@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Feiner <pfeiner@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Christoffer Dall <christoffer.dall@....com>
Subject: Re: [PATCH v2 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU
 memory cache is empty

On Mon, Jun 22, 2020 at 1:09 PM Sean Christopherson
<sean.j.christopherson@...el.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@...gle.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.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
>