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Subject: Re: [PATCH] mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased
 twice
To:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
References: <20220905133813.2253703-1-liushixin2@huawei.com>
 <20220905130728.1e814d185b189faece6f2c2f@linux-foundation.org>
 <a694528c-f572-c651-24b3-77915053f992@huawei.com>
 <20220907163532.413d52275e8a8d865e9321d2@linux-foundation.org>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
From:   Liu Shixin <liushixin2@huawei.com>
Message-ID: <6c0aeb6c-0dcc-2abb-d6af-559ff881d289@huawei.com>
Date:   Thu, 8 Sep 2022 11:28:52 +0800
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On 2022/9/8 7:35, Andrew Morton wrote:
> On Tue, 6 Sep 2022 09:52:23 +0800 Liu Shixin <liushixin2@huawei.com> wrote:
>
>> On 2022/9/6 4:07, Andrew Morton wrote:
>>> On Mon, 5 Sep 2022 21:38:13 +0800 Liu Shixin <liushixin2@huawei.com> wrote:
>>>
>>>> If two or more threads call get_huge_zero_page concurrently, THP_ZERO_PAGE_ALLOC
>>>> may increased two or more times. But actually, this should only count
>>>> as once since the extra zero pages has been freed.
>>> Well, for better of for worse,
>>> Documentation/admin-guide/mm/transhuge.rst says
>>>
>>> thp_zero_page_alloc
>>> 	is incremented every time a huge zero page is
>>> 	successfully allocated. It includes allocations which where
>>> 	dropped due race with other allocation. Note, it doesn't count
>>> 	every map of the huge zero page, only its allocation.
>>>
>>> If you think this interprtation should be changed then please explain
>>> why, and let's be sure to update the documentation accordingly.
>>>
>>> .
>> Thanks for your explanation. I misunderstood the meaning of thp_zero_page_alloc before.
>> Although the rules are clearly explained in the documentation, I think that this variable
>> should only incremented when a huge zero page used for thp is successfully allocated and
>> the pages dropped due race should skip increment. It seems strange to count in all allocations.
>>
>> If there's something I still misunderstand, please point it out, thanks.
> It seems strange to me also.  Perhaps there's a rationale buried in the
> git and mailing list history.
>
> .
I didn't find previous discussion about this point. I update document in v2.
Kirill, what do you think about this change?

https://lore.kernel.org/linux-mm/20220908035533.2186159-1-liushixin2@huawei.com/