Re: [PATCH] emulate accessed bit for EPT

[Balbir Singh]

* Rik van Riel <riel@xxxxxxxxxx> [2010-02-03 16:11:03]:

> Currently KVM pretends that pages with EPT mappings never got
> accessed.  This has some side effects in the VM, like swapping
> out actively used guest pages and needlessly breaking up actively
> used hugepages.
> 
> We can avoid those very costly side effects by emulating the
> accessed bit for EPT PTEs, which should only be slightly costly
> because pages pass through page_referenced infrequently.
> 
> TLB flushing is taken care of by kvm_mmu_notifier_clear_flush_young().
> 
> This seems to help prevent KVM guests from being swapped out when
> they should not on my system.
> 
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> Jeff, does this patch fix the issue you saw a few months ago, with
> a 256MB KVM guest in a cgroup limited to 128GB memory?
> 
>  arch/x86/kvm/mmu.c |   10 ++++++++--
>  1 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 89a49fb..6101615 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -856,9 +856,15 @@ static int kvm_age_rmapp(struct kvm *kvm, unsigned long *rmapp,
>  	u64 *spte;
>  	int young = 0;
> 
> -	/* always return old for EPT */
> +	/*
> +	 * Emulate the accessed bit for EPT, by checking if this page has
> +	 * an EPT mapping, and clearing it if it does. On the next access,
> +	 * a new EPT mapping will be established.
> +	 * This has some overhead, but not as much as the cost of swapping
> +	 * out actively used pages or breaking up actively used hugepages.
> +	 */
>  	if (!shadow_accessed_mask)
> -		return 0;
> +		return kvm_unmap_rmapp(kvm, rmapp, data);
>

Quite a clever implementation, one side effect is that one would see a
larger number of minor faults with EPT enabled and an increase in
allocation/frees of rmap entries, but that can be easily explained.

-- 
	Balbir
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