Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking

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On 06/21/2011 04:32 PM, Nai Xia wrote:
Introduced kvm_mmu_notifier_test_and_clear_dirty(), kvm_mmu_notifier_dirty_update()
and their mmu_notifier interfaces to support KSM dirty bit tracking, which brings
significant performance gain in volatile pages scanning in KSM.
Currently, kvm_mmu_notifier_dirty_update() returns 0 if and only if intel EPT is
enabled to indicate that the dirty bits of underlying sptes are not updated by
hardware.



Can you quantify the performance gains?

+int kvm_test_and_clear_dirty_rmapp(struct kvm *kvm, unsigned long *rmapp,
+				   unsigned long data)
+{
+	u64 *spte;
+	int dirty = 0;
+
+	if (!shadow_dirty_mask) {
+		WARN(1, "KVM: do NOT try to test dirty bit in EPT\n");
+		goto out;
+	}
+
+	spte = rmap_next(kvm, rmapp, NULL);
+	while (spte) {
+		int _dirty;
+		u64 _spte = *spte;
+		BUG_ON(!(_spte&  PT_PRESENT_MASK));
+		_dirty = _spte&  PT_DIRTY_MASK;
+		if (_dirty) {
+			dirty = 1;
+			clear_bit(PT_DIRTY_SHIFT, (unsigned long *)spte);
+		}

Racy.  Also, needs a tlb flush eventually.

+		spte = rmap_next(kvm, rmapp, spte);
+	}
+out:
+	return dirty;
+}
+
  #define RMAP_RECYCLE_THRESHOLD 1000


  struct mmu_notifier_ops {
+	int (*dirty_update)(struct mmu_notifier *mn,
+			     struct mm_struct *mm);
+

I prefer to have test_and_clear_dirty() always return 1 in this case (if the spte is writeable), and drop this callback.
+int __mmu_notifier_dirty_update(struct mm_struct *mm)
+{
+	struct mmu_notifier *mn;
+	struct hlist_node *n;
+	int dirty_update = 0;
+
+	rcu_read_lock();
+	hlist_for_each_entry_rcu(mn, n,&mm->mmu_notifier_mm->list, hlist) {
+		if (mn->ops->dirty_update)
+			dirty_update |= mn->ops->dirty_update(mn, mm);
+	}
+	rcu_read_unlock();
+

Should it not be &= instead?

+	return dirty_update;
+}
+
  /*
   * This function can't run concurrently against mmu_notifier_register
   * because mm->mm_users>  0 during mmu_notifier_register and exit_mmap

--
error compiling committee.c: too many arguments to function

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