Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932930Ab0BCVLS (ORCPT ); Wed, 3 Feb 2010 16:11:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44610 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932908Ab0BCVLR (ORCPT ); Wed, 3 Feb 2010 16:11:17 -0500 Date: Wed, 3 Feb 2010 16:11:03 -0500 From: Rik van Riel To: jdike@addtoit.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com, aarcange@redhat.com, mtosatti@redhat.com Subject: [PATCH] emulate accessed bit for EPT Message-ID: <20100203161103.11e2b572@annuminas.surriel.com> Organization: Red Hat, Inc. Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1825 Lines: 49 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 --- 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); spte = rmap_next(kvm, rmapp, NULL); while (spte) { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/