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

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

On Tue, Nov 05, 2019 at 03:30:00PM -0800, Dan Williams wrote:
> On Tue, Nov 5, 2019 at 3:13 PM Sean Christopherson
> <sean.j.christopherson@xxxxxxxxx> 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@xxxxxxxxxx> 
> > > 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.

But David's proposed fix for the above refcount bug is to omit the patch
so that KVM no longer treats ZONE_DEVICE pages as reserved.  That seems
like the right thing to do, including for thp_adjust(), e.g. it would
naturally let KVM use 2mb pages for the guest when a ZONE_DEVICE page is
mapped with a huge page (2mb or above) in the host.  The only hiccup is
figuring out how to correctly transfer the reference.

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