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

maobibo
2024-08-02T07:54:04Z

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@google.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:
>