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.
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