Hi Oliver,

On Tue, 6 Aug 2024 at 00:26, Oliver Upton [off-list ref] 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:

quoted

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/

quoted

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 (local)
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.

quoted

+     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