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Date:   Mon, 17 Apr 2023 12:13:52 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Wen Yang <wenyang.linux@foxmail.com>
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
Subject: Re: [PATCH] mm: compaction: optimize compact_memory to comply with
 the admin-guide
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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.

-- 
Mel Gorman
SUSE Labs