Re: [PATCH v6 5/8] KVM: x86/mmu: Track the number of TDP MMU pages, but not the actual pages

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>

On Wed, Oct 19, 2022 at 04:56:15PM +0000, Sean Christopherson wrote:
> Track the number of TDP MMU "shadow" pages instead of tracking the pages
> themselves. With the NX huge page list manipulation moved out of the common
> linking flow, elminating the list-based tracking means the happy path of
> adding a shadow page doesn't need to acquire a spinlock and can instead
> inc/dec an atomic.
> 
> Keep the tracking as the WARN during TDP MMU teardown on leaked shadow
> pages is very, very useful for detecting KVM bugs.
> 
> Tracking the number of pages will also make it trivial to expose the
> counter to userspace as a stat in the future, which may or may not be
> desirable.
> 
> Note, the TDP MMU needs to use a separate counter (and stat if that ever
> comes to be) from the existing n_used_mmu_pages. The TDP MMU doesn't bother
> supporting the shrinker nor does it honor KVM_SET_NR_MMU_PAGES (because the
> TDP MMU consumes so few pages relative to shadow paging), and including TDP
> MMU pages in that counter would break both the shrinker and shadow MMUs,
> e.g. if a VM is using nested TDP.
> 
> Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 11 +++--------
>  arch/x86/kvm/mmu/tdp_mmu.c      | 20 +++++++++-----------
>  2 files changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0333dbb8ec85..bbd2cecd34cb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1290,6 +1290,9 @@ struct kvm_arch {
>  	 */
>  	bool tdp_mmu_enabled;
>  
> +	/* The number of TDP MMU pages across all roots. */
> +	atomic64_t tdp_mmu_pages;
> +
>  	/*
>  	 * List of kvm_mmu_page structs being used as roots.
>  	 * All kvm_mmu_page structs in the list should have
> @@ -1310,18 +1313,10 @@ struct kvm_arch {
>  	 */
>  	struct list_head tdp_mmu_roots;
>  
> -	/*
> -	 * List of kvm_mmu_page structs not being used as roots.
> -	 * All kvm_mmu_page structs in the list should have
> -	 * tdp_mmu_page set and a tdp_mmu_root_count of 0.
> -	 */
> -	struct list_head tdp_mmu_pages;
> -
>  	/*
>  	 * Protects accesses to the following fields when the MMU lock
>  	 * is held in read mode:
>  	 *  - tdp_mmu_roots (above)
> -	 *  - tdp_mmu_pages (above)
>  	 *  - the link field of kvm_mmu_page structs used by the TDP MMU
>  	 *  - possible_nx_huge_pages;
>  	 *  - the possible_nx_huge_page_link field of kvm_mmu_page structs used
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 059231c82345..4e5b3ae824c1 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -29,7 +29,6 @@ int kvm_mmu_init_tdp_mmu(struct kvm *kvm)
>  	kvm->arch.tdp_mmu_enabled = true;
>  	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_roots);
>  	spin_lock_init(&kvm->arch.tdp_mmu_pages_lock);
> -	INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
>  	kvm->arch.tdp_mmu_zap_wq = wq;
>  	return 1;
>  }
> @@ -54,7 +53,7 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
>  	/* Also waits for any queued work items.  */
>  	destroy_workqueue(kvm->arch.tdp_mmu_zap_wq);
>  
> -	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_pages));
> +	WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
>  	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
>  
>  	/*
> @@ -377,11 +376,13 @@ static void handle_changed_spte_dirty_log(struct kvm *kvm, int as_id, gfn_t gfn,
>  static void tdp_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	kvm_account_pgtable_pages((void *)sp->spt, +1);
> +	atomic64_inc(&kvm->arch.tdp_mmu_pages);
>  }
>  
>  static void tdp_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>  	kvm_account_pgtable_pages((void *)sp->spt, -1);
> +	atomic64_dec(&kvm->arch.tdp_mmu_pages);
>  }
>  
>  /**
> @@ -397,17 +398,17 @@ static void tdp_mmu_unlink_sp(struct kvm *kvm, struct kvm_mmu_page *sp,
>  			      bool shared)
>  {
>  	tdp_unaccount_mmu_page(kvm, sp);
> +
> +	if (!sp->nx_huge_page_disallowed)
> +		return;
> +
>  	if (shared)
>  		spin_lock(&kvm->arch.tdp_mmu_pages_lock);
>  	else
>  		lockdep_assert_held_write(&kvm->mmu_lock);
>  
> -	list_del(&sp->link);
> -
> -	if (sp->nx_huge_page_disallowed) {
> -		sp->nx_huge_page_disallowed = false;
> -		untrack_possible_nx_huge_page(kvm, sp);
> -	}
> +	sp->nx_huge_page_disallowed = false;
> +	untrack_possible_nx_huge_page(kvm, sp);
>  
>  	if (shared)
>  		spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
> @@ -1140,9 +1141,6 @@ static int tdp_mmu_link_sp(struct kvm *kvm, struct tdp_iter *iter,
>  		tdp_mmu_set_spte(kvm, iter, spte);
>  	}
>  
> -	spin_lock(&kvm->arch.tdp_mmu_pages_lock);
> -	list_add(&sp->link, &kvm->arch.tdp_mmu_pages);
> -	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>  	tdp_account_mmu_page(kvm, sp);
>  
>  	return 0;
> -- 
> 2.38.0.413.g74048e4d9e-goog
>