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

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Message-ID: <2498F20B-DD98-4244-AD60-58D4A771ADF6@nvidia.com>
Date:   Fri, 27 Mar 2020 17:44:56 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Yang Shi <shy828301@...il.com>
CC:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across
 fork

<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.

—
Best Regards,
Yan Zi

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