has_fast_aging should be set by subscribers that non-trivially implement
fast-only versions of both test_young() and clear_young().

Fast aging must be opt-in. For a subscriber that has not been
enlightened with "fast aging", the test/clear_young() will behave
identically whether or not fast_only is given. Have KVM always opt out
for now; specific architectures can opt-in later.

Given that KVM is the only test/clear_young() implementer, we could
instead add an equivalent check in KVM, but doing so would incur an
indirect function call every time, even if the notifier ends up being a
no-op.

Add mm_has_fast_young_notifiers() in case a caller wants to know if it
should skip many calls to the mmu notifiers that may not be necessary
(like MGLRU look-around).

Signed-off-by: James Houghton <jthoughton@google.com>
---
 include/linux/mmu_notifier.h | 14 ++++++++++++++
 mm/mmu_notifier.c            | 20 ++++++++++++++++++++
 virt/kvm/kvm_main.c          |  1 +
 3 files changed, 35 insertions(+)

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index e2dd57ca368b..37643fa43687 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -231,6 +231,7 @@ struct mmu_notifier {
 	struct mm_struct *mm;
 	struct rcu_head rcu;
 	unsigned int users;
+	bool has_fast_aging;
 };
 
 /**
@@ -383,6 +384,7 @@ extern int __mmu_notifier_clear_young(struct mm_struct *mm,
 				      unsigned long end);
 extern int __mmu_notifier_test_young(struct mm_struct *mm,
 				     unsigned long address);
+extern bool __mm_has_fast_young_notifiers(struct mm_struct *mm);
 extern int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *r);
 extern void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *r);
 extern void __mmu_notifier_arch_invalidate_secondary_tlbs(struct mm_struct *mm,
@@ -428,6 +430,13 @@ static inline int mmu_notifier_test_young(struct mm_struct *mm,
 	return 0;
 }
 
+static inline bool mm_has_fast_young_notifiers(struct mm_struct *mm)
+{
+	if (mm_has_notifiers(mm))
+		return __mm_has_fast_young_notifiers(mm);
+	return 0;
+}
+
 static inline void
 mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
 {
@@ -619,6 +628,11 @@ static inline int mmu_notifier_test_young(struct mm_struct *mm,
 	return 0;
 }
 
+static inline bool mm_has_fast_young_notifiers(struct mm_struct *mm)
+{
+	return 0;
+}
+
 static inline void
 mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
 {
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 8982e6139d07..c405e5b072cf 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -382,6 +382,26 @@ int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
 	return young;
 }
 
+bool __mm_has_fast_young_notifiers(struct mm_struct *mm)
+{
+	struct mmu_notifier *subscription;
+	bool has_fast_aging = false;
+	int id;
+
+	id = srcu_read_lock(&srcu);
+	hlist_for_each_entry_rcu(subscription,
+				 &mm->notifier_subscriptions->list, hlist,
+				 srcu_read_lock_held(&srcu)) {
+		if (subscription->has_fast_aging) {
+			has_fast_aging = true;
+			break;
+		}
+	}
+	srcu_read_unlock(&srcu, id);
+
+	return has_fast_aging;
+}
+
 int __mmu_notifier_clear_young(struct mm_struct *mm,
 			       unsigned long start,
 			       unsigned long end)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7d5b35cfc1ed..f6c369eccd2a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -930,6 +930,7 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
 static int kvm_init_mmu_notifier(struct kvm *kvm)
 {
 	kvm->mmu_notifier.ops = &kvm_mmu_notifier_ops;
+	kvm->mmu_notifier.has_fast_aging = false;
 	return mmu_notifier_register(&kvm->mmu_notifier, current->mm);
 }
 
-- 
2.46.0.792.g87dc391469-goog