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

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Date:	Wed, 18 Feb 2009 15:10:17 +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 02:03:34PM +0000, Avi Kivity wrote:
> Joerg Roedel wrote:
> >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;
> >  
> 
> sp->spt is allocated using mmu_memory_cache_alloc(), which zeros the page.  How can the assertion fail?

In the code I see (current kvm-git) mmu_memory_cache_alloc() does zero
nothing. It takes the page from the preallocated pool and returns it.
The pool itself is filled with mmu_topup_memory_caches() which calls
mmu_topup_memory_cache_page() to fill the mmu_page_cache (from which the
sp->spt page is allocated later). And the mmu_topup_memory_cache_page()
function calls alloc_page() and does not zero the result. This let the
assertion trigger.

Joerg

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