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

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Nov 6, 2019 at 12:26 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Wed, Nov 06, 2019 at 10:04:22AM -0800, Dan Williams wrote:
> > On Wed, Nov 6, 2019 at 9:07 AM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > Explicitly exempt ZONE_DEVICE pages from kvm_is_reserved_pfn() and
> > > instead manually handle ZONE_DEVICE on a case-by-case basis.  For things
> > > like page refcounts, KVM needs to treat ZONE_DEVICE pages like normal
> > > pages, e.g. put pages grabbed via gup().  But KVM needs special handling
> > > in other flows where ZONE_DEVICE pages lack the underlying machinery,
> > > e.g. when setting accessed/dirty bits and shifting refcounts for
> > > transparent huge pages.
> > >
> > > This fixes a hang reported by Adam Borowski[*] in dev_pagemap_cleanup()
> > > when running a KVM guest backed with /dev/dax memory, as KVM straight up
> > > doesn't put any references to ZONE_DEVICE pages acquired by gup().
> > >
> > > [*] http://lkml.kernel.org/r/20190919115547.GA17963@angband.pl
> > >
> > > Reported-by: Adam Borowski <kilobyte@angband.pl>
> > > Debugged-by: David Hildenbrand <david@redhat.com>
> > > Cc: Dan Williams <dan.j.williams@intel.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > > ---
> > >  arch/x86/kvm/mmu.c       |  8 ++++----
> > >  include/linux/kvm_host.h |  1 +
> > >  virt/kvm/kvm_main.c      | 19 +++++++++++++++----
> > >  3 files changed, 20 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > > index 24c23c66b226..bf82b1f2e834 100644
> > > --- a/arch/x86/kvm/mmu.c
> > > +++ b/arch/x86/kvm/mmu.c
> > > @@ -3306,7 +3306,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
> > >          * here.
> > >          */
> > >         if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
> > > -           level == PT_PAGE_TABLE_LEVEL &&
> > > +           !kvm_is_zone_device_pfn(pfn) && level == PT_PAGE_TABLE_LEVEL &&
> > >             PageTransCompoundMap(pfn_to_page(pfn)) &&
> > >             !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
> > >                 unsigned long mask;
> > > @@ -5914,9 +5914,9 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> > >                  * the guest, and the guest page table is using 4K page size
> > >                  * mapping if the indirect sp has level = 1.
> > >                  */
> > > -               if (sp->role.direct &&
> > > -                       !kvm_is_reserved_pfn(pfn) &&
> > > -                       PageTransCompoundMap(pfn_to_page(pfn))) {
> > > +               if (sp->role.direct && !kvm_is_reserved_pfn(pfn) &&
> > > +                   !kvm_is_zone_device_pfn(pfn) &&
> > > +                   PageTransCompoundMap(pfn_to_page(pfn))) {
> > >                         pte_list_remove(rmap_head, sptep);
> > >
> > >                         if (kvm_available_flush_tlb_with_range())
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index a817e446c9aa..4ad1cd7d2d4d 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -966,6 +966,7 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu);
> > >  void kvm_vcpu_kick(struct kvm_vcpu *vcpu);
> > >
> > >  bool kvm_is_reserved_pfn(kvm_pfn_t pfn);
> > > +bool kvm_is_zone_device_pfn(kvm_pfn_t pfn);
> > >
> > >  struct kvm_irq_ack_notifier {
> > >         struct hlist_node link;
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index b8534c6b8cf6..0a781b1fb8f0 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -151,12 +151,23 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
> > >
> > >  bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
> > >  {
> > > +       /*
> > > +        * ZONE_DEVICE pages currently set PG_reserved, but from a refcounting
> > > +        * perspective they are "normal" pages, albeit with slightly different
> > > +        * usage rules.
> > > +        */
> > >         if (pfn_valid(pfn))
> > > -               return PageReserved(pfn_to_page(pfn));
> > > +               return PageReserved(pfn_to_page(pfn)) &&
> > > +                      !is_zone_device_page(pfn_to_page(pfn));
> >
> > This is racy unless you can be certain that the pfn and resulting page
> > has already been pinned by get_user_pages(). This is why I told David
> > to steer clear of adding more is_zone_device_page() usage, it's
> > difficult to audit. Without an existing pin the metadata to determine
> > whether a page is ZONE_DEVICE or not could be in the process of being
> > torn down.
>
> Ouch, that's brutal.  Makes perfect sense why you'd want to avoid
> is_zone_device_page().
>
> > Ideally KVM would pass around a struct { struct page *page,
> > unsigned long pfn } tuple so it would not have to do this "recall
> > context" dance on every pfn operation.
>
> Implementing something of that nature is doable, but it's going to be a
> significant overhaul as it will require updating every architecture.  So
> yeah, let's go with David's suggestion of fixing the immediate bug with
> your quick and dirty change and punting a more complete rework to future
> cleanup.  There are still multiple KVM flows that don't correctly handle
> ZONE_DEVICE, especially outside of x86, but I don't think there's a quick
> fix for those issues.

Sounds like a plan, I'll rework that fix with the change you suggested
and submit a formal one.