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@xxxxxxxxx>
>>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> ---
>>> 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
