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

[Wen Yang]

在 2023/4/12 04:48, Andrew Morton 写道:
> 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


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