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Date: Mon, 11 Nov 2019 23:20:28 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <sean.j.christopherson@...el.com>,
Radim Krčmář <rkrcmar@...hat.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, Adam Borowski <kilobyte@...band.pl>,
David Hildenbrand <david@...hat.com>,
Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 1/3] KVM: MMU: Do not treat ZONE_DEVICE pages as being
reserved
On 11/11/19 23:12, Sean Christopherson 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 for flows such as setting
> A/D bits or shifting refcounts for transparent huge pages, KVM needs to
> to avoid processing ZONE_DEVICE pages as the flows in question lack the
> underlying machinery for proper handling of ZONE_DEVICE 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().
>
> Note, Dan Williams proposed an alternative solution of doing put_page()
> on ZONE_DEVICE pages immediately after gup() in order to simplify the
> auditing needed to ensure is_zone_device_page() is called if and only if
> the backing device is pinned (via gup()). But that approach would break
> kvm_vcpu_{un}map() as KVM requires the page to be pinned from map() 'til
> unmap() when accessing guest memory, unlike KVM's secondary MMU, which
> coordinates with mmu_notifier invalidations to avoid creating stale
> page references, i.e. doesn't rely on pages being pinned.
>
> [*] http://lkml.kernel.org/r/20190919115547.GA17963@angband.pl
>
> Reported-by: Adam Borowski <kilobyte@...band.pl>
> Debugged-by: David Hildenbrand <david@...hat.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
> arch/x86/kvm/mmu.c | 8 ++++----
> include/linux/kvm_host.h | 1 +
> virt/kvm/kvm_main.c | 26 +++++++++++++++++++++++---
> 3 files changed, 28 insertions(+), 7 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..bc9d10a0a334 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -149,10 +149,30 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
> return 0;
> }
>
> +bool kvm_is_zone_device_pfn(kvm_pfn_t pfn)
> +{
> + /*
> + * The metadata used by is_zone_device_page() to determine whether or
> + * not a page is ZONE_DEVICE is guaranteed to be valid if and only if
> + * the device has been pinned, e.g. by get_user_pages(). WARN if the
> + * page_count() is zero to help detect bad usage of this helper.
> + */
> + if (!pfn_valid(pfn) || WARN_ON_ONCE(!page_count(pfn_to_page(pfn))))
> + return false;
> +
> + return is_zone_device_page(pfn_to_page(pfn));
> +}
> +
> 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)) &&
> + !kvm_is_zone_device_pfn(pfn);
>
> return true;
> }
> @@ -1865,7 +1885,7 @@ 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);
> @@ -1875,7 +1895,7 @@ 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);
>
Queued, thanks -- the other two will wait for after the merge window to
avoid pointless conflicts.
Paolo
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