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Date:   Thu, 5 Oct 2023 22:04:28 +0800
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Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when
 MEMINIT_EARLY
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To:     Mike Rapoport <rppt@kernel.org>
Cc:     David Hildenbrand <david@redhat.com>, akpm@linux-foundation.org,
        mike.kravetz@oracle.com, muchun.song@linux.dev,
        willy@infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <a6a20ff9-385c-639f-75cf-ce73a01d97cf@linux.dev>
 <20230929100252.GW3303@kernel.org>
 <15233624-f32e-172e-b2f6-7ca7bffbc96d@linux.dev>
 <20231001185934.GX3303@kernel.org>
 <90342474-432a-9fe3-2f11-915a04f0053f@linux.dev>
 <20231002084708.GZ3303@kernel.org>
 <f7e6f67a-4cac-73bd-1d5e-5020c6c8423d@redhat.com>
 <20231002111051.GA3303@kernel.org>
 <3057dab3-19f2-99ca-f125-e91a094975ed@redhat.com>
 <8c9ee3bd-6d71-4111-8f4e-91bc52b42ed4@linux.dev>
 <20231005050619.GB3303@kernel.org>
From:   Yajun Deng <yajun.deng@linux.dev>
In-Reply-To: <20231005050619.GB3303@kernel.org>
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On 2023/10/5 13:06, Mike Rapoport wrote:
> On Tue, Oct 03, 2023 at 10:38:09PM +0800, Yajun Deng wrote:
>> On 2023/10/2 19:25, David Hildenbrand wrote:
>>> On 02.10.23 13:10, Mike Rapoport wrote:
>>>>>> That 'if' breaks the invariant that __free_pages_core is
>>>>>> always called for
>>>>>> pages with initialized page count. Adding it may lead to
>>>>>> subtle bugs and
>>>>>> random memory corruption so we don't want to add it at the
>>>>>> first place.
>>>>> As long as we have to special-case memory hotplug, we know that we are
>>>>> always coming via generic_online_page() in that case. We could
>>>>> either move
>>>>> some logic over there, or let __free_pages_core() know what it
>>>>> should do.
>>>> Looks like the patch rather special cases MEMINIT_EARLY, although I
>>>> didn't
>>>> check throughfully other code paths.
>>>> Anyway, relying on page_count() to be correct in different ways for
>>>> different callers of __free_pages_core() does not sound right to me.
>>> Absolutely agreed.
>>>
>> I already sent v5  a few days ago. Comments, please...
> Does it address all the feedback from this thread?
>

Except hotplug. As far as I konw, we only clear page count in 
MEMINIT_EARLY and all tail pages in compound page.

So adding 'if (page_count(page))' will have no actual effect for other 
case. According to previous data, it didn't

become slower in hotplug.