Re: [PATCH v6 06/43] KVM: x86/mmu: Bug the VM if gmem attributes are queried to determine max mapping level

[Sean Christopherson <seanjc@google.com>]

On Wed, May 20, 2026, Ackerley Tng wrote:
> Sean Christopherson <seanjc@google.com> writes:
> >> >         } else {
> >> > +               /*
> >> > +                * Memory attributes cannot be obtained from guest_memfd while
> >> > +                * the MMU lock is held.
> >> > +                */
> >> > +               if (KVM_BUG_ON(static_call_query(__kvm_get_memory_attributes) ==
> >> > +                              kvm_gmem_get_memory_attributes, kvm)) {
> >> > +                       return 0;
> >> > +               }
> >> > +
> >>
> >> This directly takes the address of kvm_gmem_get_memory_attributes,
> >> which is only compiled if CONFIG_KVM_GUEST_MEMFD=y. This breaks
> >> ARCH=i386.
> >
> > And this bleeds guest_memfd implementation details into places they don't belong.
> > The right way to deal with this is to use lockdep_assert_not_held() in whatever
> > code mustn't run with mmu_lock held.  E.g.
> >
> > diff --git virt/kvm/guest_memfd.c virt/kvm/guest_memfd.c
> > index c9f155c2dc5c..3bea9c1137ef 100644
> > --- virt/kvm/guest_memfd.c
> > +++ virt/kvm/guest_memfd.c
> > @@ -547,6 +547,9 @@ unsigned long kvm_gmem_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
> >         struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
> >         struct inode *inode;
> >
> > +       /* Comment goes here. */
> > +       lockdep_assert_not_held(&kvm->mmu_lock);
> > +
> >         /*
> >          * If this gfn has no associated memslot, there's no chance of the gfn
> >          * being backed by private memory, since guest_memfd must be used for
> >
> > But I'm confused, because kvm_gmem_get_memory_attributes() doesn't actually take
> > filemap_invalidate_lock(), so what exactly is the problem?
> >
> 
> Ahh I can drop this patch now. kvm_gmem_get_memory_attributes() used to
> take the filemap_invalidate_lock(), but after Liam pointed out that
> the attributes maple tree should be using MT_FLAGS_USE_RCU, I stopped
> taking filemap_invalidate_lock() and forgot to undo this.
> 
> I'll wait a bit for more reviews and then put out another revision
> without this patch.

If this is the only issue with v6, don't send a new version, I'll just drop it
when applying.