linux-kernel - Re: [PATCH v12 64/84] KVM: LoongArch: Mark "struct page" pfns dirty only in "slow" page fault path

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Message-ID: <a039b758-d4e3-3798-806f-25bceb2f33a5@loongson.cn>
Date: Fri, 2 Aug 2024 15:53:48 +0800
From: maobibo <maobibo@...ngson.cn>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, Tianrui Zhao
 <zhaotianrui@...ngson.cn>, Huacai Chen <chenhuacai@...nel.org>,
 Michael Ellerman <mpe@...erman.id.au>, Anup Patel <anup@...infault.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Janosch Frank <frankja@...ux.ibm.com>,
 Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 kvmarm@...ts.linux.dev, loongarch@...ts.linux.dev,
 linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
 David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v12 64/84] KVM: LoongArch: Mark "struct page" pfns dirty
 only in "slow" page fault path



On 2024/7/27 上午7:52, Sean Christopherson wrote:
> Mark pages/folios dirty only the slow page fault path, i.e. only when
> mmu_lock is held and the operation is mmu_notifier-protected, 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
> the race is minuscule, hence the lack of complaints).
> 
> See the link below for details.
> 
> Link: https://lore.kernel.org/all/cover.1683044162.git.lstoakes@gmail.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/loongarch/kvm/mmu.c | 18 ++++++++++--------
>   1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> index 2634a9e8d82c..364dd35e0557 100644
> --- a/arch/loongarch/kvm/mmu.c
> +++ b/arch/loongarch/kvm/mmu.c
> @@ -608,13 +608,13 @@ static int kvm_map_page_fast(struct kvm_vcpu *vcpu, unsigned long gpa, bool writ
>   		if (kvm_pte_young(changed))
>   			kvm_set_pfn_accessed(pfn);
>   
> -		if (kvm_pte_dirty(changed)) {
> -			mark_page_dirty(kvm, gfn);
> -			kvm_set_pfn_dirty(pfn);
> -		}
>   		if (page)
>   			put_page(page);
>   	}
> +
> +	if (kvm_pte_dirty(changed))
> +		mark_page_dirty(kvm, gfn);
> +
>   	return ret;
>   out:
>   	spin_unlock(&kvm->mmu_lock);
> @@ -915,12 +915,14 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
>   	else
>   		++kvm->stat.pages;
>   	kvm_set_pte(ptep, new_pte);
> -	spin_unlock(&kvm->mmu_lock);
>   
> -	if (prot_bits & _PAGE_DIRTY) {
> -		mark_page_dirty_in_slot(kvm, memslot, gfn);
> +	if (writeable)
Is it better to use write or (prot_bits & _PAGE_DIRTY) here?  writable 
is pte permission from function hva_to_pfn_slow(), write is fault action.

Regards
Bibo Mao
>   		kvm_set_pfn_dirty(pfn);
> -	}
> +
> +	spin_unlock(&kvm->mmu_lock);
> +
> +	if (prot_bits & _PAGE_DIRTY)
> +		mark_page_dirty_in_slot(kvm, memslot, gfn);
>   
>   	kvm_release_pfn_clean(pfn);
>   out:
>