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Date:   Wed, 7 Sep 2022 16:35:32 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Liu Shixin <liushixin2@huawei.com>
Cc:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>
Subject: Re: [PATCH] mm/huge_memory: prevent THP_ZERO_PAGE_ALLOC increased
 twice
Message-Id: <20220907163532.413d52275e8a8d865e9321d2@linux-foundation.org>
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References: <20220905133813.2253703-1-liushixin2@huawei.com>
        <20220905130728.1e814d185b189faece6f2c2f@linux-foundation.org>
        <a694528c-f572-c651-24b3-77915053f992@huawei.com>
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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.