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

[Xiao Guangrong]

On 10/24/2013 08:32 PM, Gleb Natapov wrote:
> 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.

It's ugly but it isn't too bad. :)

Since currently kvm is extensively used to test/measure linux kernel
and the BUG_ON() in set_page_header() can help us to detect the potential
issue, it is easy for us to fix the possible bug in the development-cycle
if 'mapping' is used by slab. If that really happen, maybe we can switch
it to your way instead.


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