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Subject: Re: [PATCH] mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased
 twice
To:     Andrew Morton <akpm@linux-foundation.org>
References: <20220905133813.2253703-1-liushixin2@huawei.com>
 <20220905130728.1e814d185b189faece6f2c2f@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: <a694528c-f572-c651-24b3-77915053f992@huawei.com>
Date:   Tue, 6 Sep 2022 09:52:23 +0800
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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.

Thanks,