linux-kernel - Re: [PATCH] KVM: x86/mmu: Grab nx_lpage_splits as an unsigned long before division

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Message-ID: <83be37371a61efa72c4364f1dc9a26d19750d8cc.camel@redhat.com>
Date:   Wed, 16 Jun 2021 22:01:57 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Grab nx_lpage_splits as an unsigned long
 before division

On Tue, 2021-06-15 at 09:29 -0700, Sean Christopherson wrote:
> Snapshot kvm->stats.nx_lpage_splits into a local unsigned long to avoid
> 64-bit division on 32-bit kernels.  Casting to an unsigned long is safe
> because the maximum number of shadow pages, n_max_mmu_pages, is also an
> unsigned long, i.e. KVM will start recycling shadow pages before the
> number of splits can exceed a 32-bit value.
> 
>   ERROR: modpost: "__udivdi3" [arch/x86/kvm/kvm.ko] undefined!
> 
> Fixes: 7ee093d4f3f5 ("KVM: switch per-VM stats to u64")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 720ceb0a1f5c..7d3e57678d34 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6043,6 +6043,7 @@ static int set_nx_huge_pages_recovery_ratio(const char *val, const struct kernel
>  
>  static void kvm_recover_nx_lpages(struct kvm *kvm)
>  {
> +	unsigned long nx_lpage_splits = kvm->stat.nx_lpage_splits;
>  	int rcu_idx;
>  	struct kvm_mmu_page *sp;
>  	unsigned int ratio;
> @@ -6054,7 +6055,7 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
>  	write_lock(&kvm->mmu_lock);
>  
>  	ratio = READ_ONCE(nx_huge_pages_recovery_ratio);
> -	to_zap = ratio ? DIV_ROUND_UP(kvm->stat.nx_lpage_splits, ratio) : 0;
> +	to_zap = ratio ? DIV_ROUND_UP(nx_lpage_splits, ratio) : 0;
>  	for ( ; to_zap; --to_zap) {
>  		if (list_empty(&kvm->arch.lpage_disallowed_mmu_pages))
>  			break;
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky