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

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Date:	Thu, 24 Oct 2013 19:01:49 +0800
From:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:	Gleb Natapov <gleb@...hat.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
Subject: Re: [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab

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.


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