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From:   Zi Yan <ziy@nvidia.com>
To:     Yang Shi <shy828301@gmail.com>
CC:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across
 fork
Date:   Fri, 27 Mar 2020 17:44:56 -0400
Message-ID: <2498F20B-DD98-4244-AD60-58D4A771ADF6@nvidia.com>
In-Reply-To: <CAHbLzkrLxB93xs78xejsBsZ=-b+oqNao6a5Bydi8-6+wpzjKQg@mail.gmail.com>
References: <20200327170601.18563-1-kirill.shutemov@linux.intel.com>
 <20200327170601.18563-5-kirill.shutemov@linux.intel.com>
 <AD80A72A-8FB7-4F89-A2C9-CDD5C616A479@sent.com>
 <CAHbLzkrLxB93xs78xejsBsZ=-b+oqNao6a5Bydi8-6+wpzjKQg@mail.gmail.com>
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<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) !=3D 1 + PageSwapCache(page)) {
>>> +             if (total_mapcount(page) + PageSwapCache(page) !=3D
>>> +                             page_count(page)) {
>>>                       unlock_page(page);
>>>                       result =3D SCAN_PAGE_COUNT;
>>>                       goto out;
>>> @@ -680,7 +688,6 @@ static void __collapse_huge_page_copy(pte_t *pte,=
 struct page *page,
>>>               } else {
>>>                       src_page =3D pte_page(pteval);
>>>                       copy_user_highpage(page, src_page, address, vma=
);
>>> -                     VM_BUG_ON_PAGE(page_mapcount(src_page) !=3D 1, =
src_page);
>>
>> Maybe replace it with this?
>>
>> VM_BUG_ON_PAGE(page_mapcount(src_page) + PageSwapCache(src_page) !=3D =
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.

=E2=80=94
Best Regards,
Yan Zi

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