From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
"Vitaly Kuznetsov" <vkuznets@redhat.com>,
"Wanpeng Li" <wanpengli@tencent.com>,
"Jim Mattson" <jmattson@google.com>,
"Joerg Roedel" <joro@8bytes.org>,
"KVM list" <kvm@vger.kernel.org>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
"Adam Borowski" <kilobyte@angband.pl>,
"David Hildenbrand" <david@redhat.com>
Subject: Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved
Date: Wed, 6 Nov 2019 12:26:27 -0800 [thread overview]
Message-ID: <20191106202627.GF16249@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4gJk2cXLdT2dZwCH2AssMVNxUfdx-bYYwJwy1LwFxOs0w@mail.gmail.com>
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.
next prev parent reply other threads:[~2019-11-06 20:26 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-06 17:07 [PATCH 0/2] KVM: MMU: Fix a refcount bug with ZONE_DEVICE pages Sean Christopherson
2019-11-06 17:07 ` [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved Sean Christopherson
2019-11-06 17:14 ` Paolo Bonzini
2019-11-06 17:46 ` Sean Christopherson
2019-11-06 18:04 ` Dan Williams
2019-11-06 20:26 ` Sean Christopherson [this message]
2019-11-06 20:34 ` Dan Williams
2019-11-06 21:09 ` Paolo Bonzini
2019-11-06 21:30 ` Sean Christopherson
2019-11-06 23:20 ` Dan Williams
2019-11-06 23:39 ` Sean Christopherson
2019-11-07 0:01 ` Dan Williams
2019-11-07 5:48 ` Dan Williams
2019-11-07 11:12 ` Paolo Bonzini
2019-11-07 15:36 ` Dan Williams
2019-11-07 15:58 ` Sean Christopherson
2019-11-09 1:43 ` Sean Christopherson
2019-11-09 2:00 ` Dan Williams
2019-11-11 18:27 ` Sean Christopherson
2019-11-11 19:15 ` Paolo Bonzini
2019-11-12 0:51 ` Dan Williams
2019-11-12 10:19 ` Paolo Bonzini
2019-11-12 16:57 ` Sean Christopherson
2019-11-06 17:07 ` [PATCH 2/2] KVM: x86/mmu: Add helper to consolidate huge page promotion Sean Christopherson
2019-11-06 17:22 ` Paolo Bonzini
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