On Wed, Nov 6, 2019 at 9:07 AM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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@xxxxxxxxxx > > Reported-by: Adam Borowski <kilobyte@xxxxxxxxxx> > Debugged-by: David Hildenbrand <david@xxxxxxxxxx> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > --- > 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.
