Re: [PATCH] mm: compaction: optimize compact_memory to comply with the admin-guide

[Wen Yang]

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@xxxxxxxxxxx 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