Re: [PATCH v2 1/3] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved

[Paolo Bonzini]

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@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      | 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