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 07:00 Mike Kravetz wrote:
>On 1/9/20 2:48 PM, Li Xinhai wrote:
>> 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.
>
>Thank you.  I think we all agree on what the proposed code is doing.
>However, we would like to know why you believe this code should be added.
>For example,
>- Did you actually encounter this situation (PageHuge(hpage) && pfn !=
>  hpage_pfn)?
>- Did you discover some code path where we are likely to encounter this
>  situation?
>- Some other reason? 

I didn't actually encounter this condition.

There are two ways for faulty code,
1. one is from the 'hpage', it could be head or tail page of hugetlbfs (I see that
current code make sure always call with head page as you mentioned). Luckly,
we catch the tail page case as BUG at begining of this mapped_walk(the
page_hstate(page) return NULL for tail page).
2. The other is from the content stored in the PTE, wihch we used as 'pfn' and
compare with 'hpage'.

Current code matches 'pfn' and 'hpage' like below:
- normal 4k page: hpage_pfn <= pfn < hpage_pfn + 1
- THP, hugetlbfs page:  hpage_pfn <= pfn < hpage_pfn + HPAGE_PMD_NR
we need do exact match for normal 4K page and hugetlbfs page, and range
match for THP.

>--
>Mike Kravetz