On 05/28/2012 04:41 PM, Xiao Guangrong wrote:
> On 05/28/2012 09:14 PM, Avi Kivity wrote:
>
>> 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?
>>
>
>
> I think the reason we move refcount in current code is, we should increase the
> refcount of the page we will mapped into shadow page table, since we always
> decrease its refcount after it is mapped. (That is this patch does.)
>
As far as I can tell __get_user_pages_fast() will take the reference
count in the page head in the first place.
--
error compiling committee.c: too many arguments to function
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