Re: [PATCH 5/9] migrate: enable migrate_pages() to migrate hugepage

[Simon Jeons <simon.jeons@gmail.com>]

Hi Naoya,
On 03/21/2013 05:59 AM, Naoya Horiguchi wrote:
> On Wed, Mar 20, 2013 at 08:31:06AM +0800, Simon Jeons wrote:
> ...
>>>>> diff --git v3.8.orig/mm/mempolicy.c v3.8/mm/mempolicy.c
>>>>> index e2df1c1..8627135 100644
>>>>> --- v3.8.orig/mm/mempolicy.c
>>>>> +++ v3.8/mm/mempolicy.c
>>>>> @@ -525,6 +525,27 @@ static int check_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>  	return addr != end;
>>>>>  }
>>>>>  
>>>>> +static void check_hugetlb_pmd_range(struct vm_area_struct *vma, pmd_t *pmd,
>>>>> +		const nodemask_t *nodes, unsigned long flags,
>>>>> +				    void *private)
>>>>> +{
>>>>> +#ifdef CONFIG_HUGETLB_PAGE
>>>>> +	int nid;
>>>>> +	struct page *page;
>>>>> +
>>>>> +	spin_lock(&vma->vm_mm->page_table_lock);
>>>>> +	page = pte_page(huge_ptep_get((pte_t *)pmd));
>>>>> +	spin_unlock(&vma->vm_mm->page_table_lock);
>>>> I am a bit confused why page_table_lock is used here and why it doesn't
>>>> cover the page usage.
>>> I expected this function to do the same for pmd as check_pte_range() does
>>> for pte, but the above code didn't do it. I should've put spin_unlock
>>> below migrate_hugepage_add(). Sorry for the confusion.
>> I still confuse! Could you explain more in details?
> With the above code, check_hugetlb_pmd_range() checks page_mapcount
> outside the page table lock, but mapcount can be decremented by
> __unmap_hugepage_range(), so there's a race.
> __unmap_hugepage_range() calls page_remove_rmap() inside page table lock,
> so we can avoid this race by doing whole check_hugetlb_pmd_range()'s work
> inside the page table lock.

Why you use page_table_lock instead of split ptlock to protect 2MB?

>
> Thanks,
> Naoya