linux-kernel - Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT

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Message-ID: <6545ac4c-1205-6c09-49ea-e00c24d1a2ff@linux.dev>
Date:   Thu, 5 Oct 2023 22:04:28 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, muchun.song@...ux.dev,
        willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm: Init page count in reserve_bootmem_region when
 MEMINIT_EARLY


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.