On 05/28/2012 03:56 PM, Xiao Guangrong wrote:
> On 05/28/2012 08:24 PM, Avi Kivity wrote:
>
>> On 05/28/2012 02:39 PM, Xiao Guangrong wrote:
>>> On 05/28/2012 06:57 PM, Avi Kivity wrote:
>>>
>>>> On 05/28/2012 09:10 AM, Xiao Guangrong wrote:
>>>>> The huge page size is 4M on non-PAE host, but 2M page size is used in
>>>>> transparent_hugepage_adjust(), so the page we get after adjust the
>>>>> mapping level is not the head page, the BUG_ON() will be triggered
>>>>>
>>>>>
>>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>>> index 72102e0..be3cea4 100644
>>>>> --- a/arch/x86/kvm/mmu.c
>>>>> +++ b/arch/x86/kvm/mmu.c
>>>>> @@ -2595,8 +2595,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
>>>>> *gfnp = gfn;
>>>>> kvm_release_pfn_clean(pfn);
>>>>> pfn &= ~mask;
>>>>> - if (!get_page_unless_zero(pfn_to_page(pfn)))
>>>>> - BUG();
>>>>> + kvm_get_pfn(pfn);
>>>>> *pfnp = pfn;
>>>>> }
>>>>> }
>>>>
>>>> Shouldn't we adjust mask instead?
>>>>
>>>
>>>
>>> Adjusting mask to map the whole 4M huge page to KVM guest?
>>
>> The code moves the refcount from the small page to the huge page. i.e.
>> from pfn 0x1312 to pfn 0x1200. But if the huge page frame contains
>> 0x400 pages, it should move the refcount to pfn 0x1000.
>>
>
>
> We need not move the refcount to the huge page (the head of pages), moving
> the refcount to the any middle small page is also ok, get_page() will
> properly handle it:
>
> get_page() -> __get_page_tail():
>
> | struct page *page_head = compound_trans_head(page);
> |
> | if (likely(page != page_head && get_page_unless_zero(page_head))) {
> | /*
> | * page_head wasn't a dangling pointer but it
> | * may not be a head page anymore by the time
> | * we obtain the lock. That is ok as long as it
> | * can't be freed from under us.
> | */
> | flags = compound_lock_irqsave(page_head);
> | /* here __split_huge_page_refcount won't run anymore */
> | if (likely(PageTail(page))) {
> | __get_page_tail_foll(page, false);
> | got = true;
> | }
> | compound_unlock_irqrestore(page_head, flags);
> | if (unlikely(!got))
> | put_page(page_head);
> | }
>
> The refcount of page_head is increased.
>
So, the whole thing is unneeded? Andrea?
--
error compiling committee.c: too many arguments to function
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