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    Subject[PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved
    Date
    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));

    return true;
    }

    +bool kvm_is_zone_device_pfn(kvm_pfn_t pfn)
    +{
    + return pfn_valid(pfn) && is_zone_device_page(pfn_to_page(pfn));
    +}
    +
    /*
    * Switches to specified vcpu, until a matching vcpu_put()
    */
    @@ -1865,7 +1876,7 @@ EXPORT_SYMBOL_GPL(kvm_release_pfn_dirty);

    void kvm_set_pfn_dirty(kvm_pfn_t pfn)
    {
    - if (!kvm_is_reserved_pfn(pfn)) {
    + if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) {
    struct page *page = pfn_to_page(pfn);

    SetPageDirty(page);
    @@ -1875,14 +1886,14 @@ EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty);

    void kvm_set_pfn_accessed(kvm_pfn_t pfn)
    {
    - if (!kvm_is_reserved_pfn(pfn))
    + if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
    mark_page_accessed(pfn_to_page(pfn));
    }
    EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);

    void kvm_get_pfn(kvm_pfn_t pfn)
    {
    - if (!kvm_is_reserved_pfn(pfn))
    + if (!kvm_is_reserved_pfn(pfn) && !WARN_ON(kvm_is_zone_device_pfn(pfn)))
    get_page(pfn_to_page(pfn));
    }
    EXPORT_SYMBOL_GPL(kvm_get_pfn);
    --
    2.24.0
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