* Rik van Riel <r...@redhat.com> [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 <r...@redhat.com>
> ---
> 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
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
