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

Fuad Tabba
2024-08-06T08:25:46Z

Hi Oliver,

On Tue, 6 Aug 2024 at 00:26, Oliver Upton <oliver.upton@linux.dev> wrote:
>
> [+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.

That was for some refactoring I'd done later on for mem_aborts in
pKVM. That said, I didn't know at the time that there might be a race
with some filesystems. I'll keep this in mind for the pKVM code we
have for now, and when upstreaming.

Thanks,
/fuad

> 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
>