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

Oliver Upton
2024-08-05T23:26:20Z

[+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/cover.1683044162.git.lstoakes@gmail.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> 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