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Date:   Tue, 18 Apr 2023 22:10:32 +0800
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Subject: Re: [PATCH] mm: compaction: optimize compact_memory to comply with
 the admin-guide
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        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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 <20230417111352.v26slrcmz4qo3tnn@techsingularity.net>
From:   Wen Yang <wenyang.linux@foxmail.com>
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在 2023/4/17 19:13, Mel Gorman 写道:
> On Sun, Apr 16, 2023 at 01:42:44AM +0800, Wen Yang wrote:
>> ??? 2023/4/13 00:54, Wen Yang ??????:
>>> ??? 2023/4/12 04:48, Andrew Morton ??????:
>>>> 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?
>>
> Enforce the 1 on the unlikely chance that the sysctl handler is ever
> extended to do something different and expects a bitmask. The original
> intent intent of the sysctl was debugging -- demonstrating a contiguous
> allocation failure when aggressive compaction should have succeeded. Later
> some machines dedicated to batch jobs used the compaction sysctl to compact
> memory before a new job started to reduce startup latencies.
>
> Drop the justification "In some critical scenarios, some applications
> operating it, such as echo 0, have caused serious problems." from the
> changelog. I cannot imagine a sane "critical scenario" where an application
> running as root is writing expected garbage to proc or sysfs files and
> then surprised when something unexpected happens.
>
Thanks for your comments.

We will modify it according to your suggestion and then send v2.


--

Best wishes,

Wen