linux-kernel - Re: [PATCH 3/3] kvm mmu: alloc shadow pages with __GFP

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Message-ID: <20090218135436.GF8751@amd.com>
Date:	Wed, 18 Feb 2009 14:54:37 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] kvm mmu: alloc shadow pages with __GFP_ZERO

On Wed, Feb 18, 2009 at 01:47:04PM +0000, Avi Kivity wrote:
> Joerg Roedel wrote:
> >Not using __GFP_ZERO when allocating shadow pages triggers the
> >assertion in the kvm_mmu_alloc_page() when MMU debugging is enabled.
> >
> >Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> >---
> > arch/x86/kvm/mmu.c |    2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> >diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> >index c90b4b2..d93ecec 100644
> >--- a/arch/x86/kvm/mmu.c
> >+++ b/arch/x86/kvm/mmu.c
> >@@ -301,7 +301,7 @@ static int mmu_topup_memory_cache_page(struct kvm_mmu_memory_cache *cache,
> > 	if (cache->nobjs >= min)
> > 		return 0;
> > 	while (cache->nobjs < ARRAY_SIZE(cache->objects)) {
> >-		page = alloc_page(GFP_KERNEL);
> >+		page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> > 		if (!page)
> > 			return -ENOMEM;
> > 		set_page_private(page, 0);
> >  
> 
> What is the warning?
> 
> Adding __GFP_ZERO here will cause us to clear the page twice, which is wasteful.

The assertion which the attached patch removes fails sometimes. Removing
this assertion is the alternative solution to this problem ;-)

>From ca45f3a2e45cd7e76ca624bb1098329db8ff83ab Mon Sep 17 00:00:00 2001
From: Joerg Roedel <joerg.roedel@....com>
Date: Wed, 18 Feb 2009 14:51:13 +0100
Subject: [PATCH] kvm mmu: remove assertion in kvm_mmu_alloc_page

Signed-off-by: Joerg Roedel <joerg.roedel@....com>
---
 arch/x86/kvm/mmu.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index d93ecec..b226973 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -802,7 +802,6 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
 	set_page_private(virt_to_page(sp->spt), (unsigned long)sp);
 	list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);
 	INIT_LIST_HEAD(&sp->oos_link);
-	ASSERT(is_empty_shadow_page(sp->spt));
 	bitmap_zero(sp->slot_bitmap, KVM_MEMORY_SLOTS + KVM_PRIVATE_MEM_SLOTS);
 	sp->multimapped = 0;
 	sp->parent_pte = parent_pte;
-- 
1.5.6.4

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