Re: [PATCH v2] mm/page_vma_mapped.c: Detect mismatched pfn of hugetlbfs page in pfn_in_hpage()

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-01-10 at 01:09 Mike Kravetz wrote:
>On 1/9/20 7:00 AM, Michal Hocko wrote:
>> [Cc Mike who is hugetlb maintainer]
>>
>> On Thu 09-01-20 14:26:02, Li Xinhai wrote:
>>> check_pte is called for hugetlbfs page and comparing pfn in pfn_in_page,
>>> where pfn is compared in range [hpage_pfn, hpage_pfn+HPAGE_PMD_NR). For
>>> hugetlbfs page, pfn must equal to hpage_pfn, so catch it as bug if they
>>> are not equal.
>>
>> Why do we need this? Would that help to debug any past bug?
>
>I have the same question as Michal.  Did you see an issue where such a check
>could help?
>
>There is nothing wrong with the proposed code.  However, it would be good to
>know the reason for adding it.  I started looking at all possible code paths
>where this could apply.  However, it would be better if you pointed out the
>concern that caused you to create the patch.
>--
>Mike Kravetz
> 
oops, I didn't write the code correctly. I should wrote it as 

	if (pfn >= hpage_pfn && pfn - hpage_pfn < hpage_nr_pages(hpage)) {
		VM_BUG_ON_PAGE(PageHuge(hpage) && pfn != hpage_pfn, hpage);
		return true;
	}

	return false;

hpage_nr_pages(hpage) give us HPAGE_PMD_NR for THP and hugetlbfs page,
but remapping PTE to a differrnt hugetlbfs page still allowed, so put the BUG code
into this condition is necessary. By this way, if it was not a exact match for PageHuge,
then it is a bug.

>>
>>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>> ---
>>>  mm/page_vma_mapped.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>>> index eff4b45..713aaed 100644
>>> --- a/mm/page_vma_mapped.c
>>> +++ b/mm/page_vma_mapped.c
>>> @@ -55,6 +55,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
>>>  static inline bool pfn_in_hpage(struct page *hpage, unsigned long pfn)
>>>  {
>>>  unsigned long hpage_pfn = page_to_pfn(hpage);
>>> +	VM_BUG_ON_PAGE(PageHuge(hpage) && pfn != hpage_pfn, hpage);
>>> 
>>>  /* THP can be referenced by any subpage */
>>>  return pfn >= hpage_pfn && pfn - hpage_pfn < hpage_nr_pages(hpage);
>>> --
>>> 1.8.3.1