Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across fork

[Zi Yan]

<snip>
>>>                       /*
>>>                        * Drain pagevec and retry just in case we can get rid
>>>                        * of the extra pin, like in swapin case.
>>>                        */
>>>                       lru_add_drain();
>>>               }
>>> -             if (page_count(page) != 1 + PageSwapCache(page)) {
>>> +             if (total_mapcount(page) + PageSwapCache(page) !=
>>> +                             page_count(page)) {
>>>                       unlock_page(page);
>>>                       result = SCAN_PAGE_COUNT;
>>>                       goto out;
>>> @@ -680,7 +688,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
>>>               } else {
>>>                       src_page = pte_page(pteval);
>>>                       copy_user_highpage(page, src_page, address, vma);
>>> -                     VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
>>
>> Maybe replace it with this?
>>
>> VM_BUG_ON_PAGE(page_mapcount(src_page) + PageSwapCache(src_page) != page_count(src_page), src_page);
>
> I don't think this is correct either. If a THP is PTE mapped its
> refcount would be bumped by the number of PTE mapped subpages. But
> page_mapcount() would just return the mapcount of that specific
> subpage. So, total_mapcount() should be used, but the same check has
> been done before reaching here.

Yes, you are right. Thanks.

Please disregard this comment.

â
Best Regards,
Yan Zi

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