[PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes

[Dan Williams]

On Tue, Nov 5, 2019 at 3:13 PM Sean Christopherson
<sean.j.christopherson at intel.com> wrote:
>
> On Tue, Nov 05, 2019 at 03:02:40PM -0800, Dan Williams wrote:
> > On Tue, Nov 5, 2019 at 12:31 PM David Hildenbrand <david at redhat.com> wrote:
> > > > The scarier code (for me) is transparent_hugepage_adjust() and
> > > > kvm_mmu_zap_collapsible_spte(), as I don't at all understand the
> > > > interaction between THP and _PAGE_DEVMAP.
> > >
> > > The x86 KVM MMU code is one of the ugliest code I know (sorry, but it
> > > had to be said :/ ). Luckily, this should be independent of the
> > > PG_reserved thingy AFAIKs.
> >
> > Both transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte()
> > are honoring kvm_is_reserved_pfn(), so again I'm missing where the
> > page count gets mismanaged and leads to the reported hang.
>
> When mapping pages into the guest, KVM gets the page via gup(), which
> increments the page count for ZONE_DEVICE pages.  But KVM puts the page
> using kvm_release_pfn_clean(), which skips put_page() if PageReserved()
> and so never puts its reference to ZONE_DEVICE pages.

Oh, yeah, that's busted.

> My transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte()
> comments were for a post-patch/series scenario wheren PageReserved() is
> no longer true for ZONE_DEVICE pages.

Ah, ok, for that David is preserving kvm_is_reserved_pfn() returning
true for ZONE_DEVICE because pfn_to_online_page() will fail for
ZONE_DEVICE.