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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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.

>> But it seams 4M page size is not supported on VMX/SVM.
> 
> We always use 64-bit PTEs in the lowest level, whether using shadow,
> EPT, or NPT.  Note NPT supports 32-bit PTEs in the lowest level, but we
> don't support that configuration.  But that doesn't mean we can't use
> host 4M pages to back guest 2M pages (or direct maps).
> 


Right.