linux-kernel - Re: [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab

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Date:	Thu, 24 Oct 2013 15:32:39 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	aarcange@...hat.com
Subject: Re: [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab

On Thu, Oct 24, 2013 at 07:01:49PM +0800, Xiao Guangrong wrote:
> On 10/24/2013 06:39 PM, Gleb Natapov wrote:
> > On Thu, Oct 24, 2013 at 06:10:46PM +0800, Xiao Guangrong wrote:
> >> On 10/24/2013 05:52 PM, Gleb Natapov wrote:
> >>> On Thu, Oct 24, 2013 at 05:29:44PM +0800, Xiao Guangrong wrote:
> >>>> On 10/24/2013 05:19 PM, Gleb Natapov wrote:
> >>>>
> >>>>>> @@ -946,7 +947,7 @@ static inline struct kvm_mmu_page *page_header(hpa_t shadow_page)
> >>>>>>  {
> >>>>>>  	struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
> >>>>>>  
> >>>>>> -	return (struct kvm_mmu_page *)page_private(page);
> >>>>>> +	return (struct kvm_mmu_page *)(page->mapping);
> >>>>> Why?
> >>>>
> >>>> That's because page->private has been used by slab:
> >>>>
> >>> But does lockless path actually looks at it?
> >>
> >> Lockless path does not use it, however, it is used by kvm_mmu_page():
> >>
> >> static inline struct kvm_mmu_page *page_header(hpa_t shadow_page)
> >> {
> >> 	struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
> >>
> >> 	return (struct kvm_mmu_page *)(page->mapping);
> >> }
> >>
> >> which is used in the common code.
> > Ah, so the pointer is not available even after object is allocated.
> > Make sense since we allocate object, not page here, but is it safe to
> > use mapping like that?
> 
> The commens says:
> 
> 	struct address_space *mapping;	/* If low bit clear, points to
> 					 * inode address_space, or NULL.
> 					 * If page mapped as anonymous
> 					 * memory, low bit is set, and
> 					 * it points to anon_vma object:
> 					 * see PAGE_MAPPING_ANON below.
> 
> It seems mapping is used for address_space or anonymous memory, in
> our case, the page is used by slab, so I guess it is ok. And the bug
> i put in set_page_header() was not tiggered on both slab and slub.
> 
Yeah, I also think so. I asked Andrea (copied) and he thinks that it is
safe too currently, but things changes fast in this area. Andrea?
Another option is too save slab_cache pointer and reset it before
freeing the object but it looks ugly.

--
			Gleb.
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