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Date:   Sun, 16 Apr 2023 01:42:44 +0800
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Subject: Re: [PATCH] mm: compaction: optimize compact_memory to comply with
 the admin-guide
From:   Wen Yang <wenyang.linux@foxmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Oscar Salvador <osalvador@suse.de>,
        William Lam <william.lam@bytedance.com>,
        Fu Wei <wefu@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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在 2023/4/13 00:54, Wen Yang 写道:
>
> 在 2023/4/12 04:48, Andrew Morton 写道:
>> On Wed, 12 Apr 2023 02:24:26 +0800 wenyang.linux@foxmail.com wrote:
>>
>>> For the /proc/sys/vm/compact_memory file, the admin-guide states:
>>> When 1 is written to the file, all zones are compacted such that free
>>> memory is available in contiguous blocks where possible. This can be
>>> important for example in the allocation of huge pages although 
>>> processes
>>> will also directly compact memory as required
>>>
>>> But it was not strictly followed, writing any value would cause all
>>> zones to be compacted. In some critical scenarios, some applications
>>> operating it, such as echo 0, have caused serious problems.
>> Really?  You mean someone actually did this and didn't observe the
>> effect during their testing?
>
> Thanks for your reply.
>
> Since /proc/sys/vm/compact_memory has been well documented for over a 
> decade:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Documentation/admin-guide/sysctl/vm.rst#n109 
>
>
> it is believed that only writing 1 will trigger trigger all zones to 
> be compacted.
>
> Especially for those who write applications, they may only focus on 
> documentation and generally do not read kernel code.  Moreover, such 
> problems are not easily detected through testing on low pressure 
> machines.
>
> Writing any meaningful or meaningless values will trigger it and 
> affect the entire server:
>
> # echo 1 > /proc/sys/vm/compact_memory
> # echo 0 > /proc/sys/vm/compact_memory
> # echo dead > /proc/sys/vm/compact_memory
> # echo "hello world" > /proc/sys/vm/compact_memory
>
> The implementation of this high-risk operation may require following 
> the admin-guides.
>
> -- 
>
> Best wishes,
>
> Wen
>
>
Hello, do you think it's better to optimize the 
sysctl_compaction_handler code or update the admin-guide document?

--

Best wishes,

Wen

>>> It has been slightly optimized to comply with the admin-guide.
>