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 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. 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.