linux-kernel - Re: [PATCH] KVM: MMU: fix huge page adapted on non-PAE host

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Date:	Mon, 28 May 2012 16:53:38 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] KVM: MMU: fix huge page adapted on non-PAE host

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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