Subject: Re: [PATCH v12 54/84] KVM: arm64: Mark "struct page" pfns accessed/dirty before dropping mmu_lock

From: Oliver Upton <[email-masked]>
Date: Mon, 5 Aug 2024 23:25:55 +0000

[+cc Fuad]

Fuad, you mentioned in commit 9c30fc615daa ("KVM: arm64: Move setting
the page as dirty out of the critical section") that restructuring
around the MMU lock was helpful for reuse (presumably for pKVM), but I
lack the context there.

On Fri, Jul 26, 2024 at 04:52:03PM -0700, Sean Christopherson wrote:
> Mark pages/folios accessed+dirty prior to dropping mmu_lock, as marking a
> page/folio dirty after it has been written back can make some filesystems
> unhappy (backing KVM guests will such filesystem files is uncommon, and

typo: s/will/with/

> the race is minuscule, hence the lack of complaints).  See the link below
> for details.
> 
> This will also allow converting arm64 to kvm_release_faultin_page(), which
> requires that mmu_lock be held (for the aforementioned reason).
> 
> Link: https://lore.kernel.org/all/[email-masked]
> Signed-off-by: Sean Christopherson <[email-masked]>
> ---
>  arch/arm64/kvm/mmu.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 22ee37360c4e..ce13c3d884d5 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1685,15 +1685,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	}
>  
>  out_unlock:
> +	if (writable && !ret)
> +		kvm_set_pfn_dirty(pfn);

I'm guessing you meant kvm_release_pfn_dirty() here, because this leaks
a reference.

> +	else
> +		kvm_release_pfn_clean(pfn);
> +
>  	read_unlock(&kvm->mmu_lock);
>  
>  	/* Mark the page dirty only if the fault is handled successfully */
> -	if (writable && !ret) {
> -		kvm_set_pfn_dirty(pfn);
> +	if (writable && !ret)
>  		mark_page_dirty_in_slot(kvm, memslot, gfn);
> -	}
>  
> -	kvm_release_pfn_clean(pfn);
>  	return ret != -EAGAIN ? ret : 0;
>  }
>  
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
> 

-- 
Thanks,
Oliver