On Wed, Nov 6, 2019 at 12:26 PM Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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@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. > > 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.
