Re: [PATCH 3/3] kvm mmu: alloc shadow pages with __GFP_ZERO

[Joerg Roedel]

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@xxxxxxx>
> >---
> > 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 ;-)