linux-kernel - Re: [PATCH v4 4/4] KVM: mmu: remove over-aggressive warnings

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Date:   Wed, 13 Oct 2021 00:02:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Stevens <stevensd@...omium.org>
Cc:     Marc Zyngier <maz@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will@...nel.org>,
        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-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v4 4/4] KVM: mmu: remove over-aggressive warnings

On Wed, Sep 29, 2021, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 
> Remove two warnings that require ref counts for pages to be non-zero, as
> mapped pfns from follow_pfn may not have an initialized ref count.
> 
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
>  arch/x86/kvm/mmu/mmu.c | 7 -------
>  virt/kvm/kvm_main.c    | 2 +-
>  2 files changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 5a1adcc9cfbc..3b469df63bcf 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -617,13 +617,6 @@ static int mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
>  
>  	pfn = spte_to_pfn(old_spte);
>  
> -	/*
> -	 * KVM does not hold the refcount of the page used by
> -	 * kvm mmu, before reclaiming the page, we should
> -	 * unmap it from mmu first.
> -	 */
> -	WARN_ON(!kvm_is_reserved_pfn(pfn) && !page_count(pfn_to_page(pfn)));

Have you actually observed false positives with this WARN?  I would expect anything
without a struct page to get filtered out by !kvm_is_reserved_pfn(pfn).

If you have observed false positives, I would strongly prefer we find a way to
keep the page_count() sanity check, it has proven very helpful in the past in
finding/debugging bugs during MMU development.

> -
>  	if (is_accessed_spte(old_spte))
>  		kvm_set_pfn_accessed(pfn);
>