Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved

[Dan Williams]

On Wed, Nov 6, 2019 at 4:01 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Wed, Nov 6, 2019 at 3:39 PM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> wrote:
> >
> > On Wed, Nov 06, 2019 at 03:20:11PM -0800, Dan Williams wrote:
> > > After some more thought I'd feel more comfortable just collapsing the
> > > ZONE_DEVICE case into the VM_IO/VM_PFNMAP case. I.e. with something
> > > like this (untested) that just drops the reference immediately and let
> > > kvm_is_reserved_pfn() do the right thing going forward.
> >
> > This will break the page fault flow, as it will allow the page to be
> > whacked before KVM can ensure it will get proper notification from the
> > mmu_notifier.  E.g. KVM would install the PFN in its secondary MMU after
> > getting the invalidate notification for the PFN.
>
> How do mmu notifiers get held off by page references and does that
> machinery work with ZONE_DEVICE? Why is this not a concern for the
> VM_IO and VM_PFNMAP case?

Put another way, I see no protection against truncate/invalidate
afforded by a page pin. If you need guarantees that the page remains
valid in the VMA until KVM can install a mmu notifier that needs to
happen under the mmap_sem as far as I can see. Otherwise gup just
weakly asserts "this pinned page was valid in this vma at one point in
time".