Re: [PATCH] memcg: change behavior of moving charges at task move

Andrew Morton
2012-03-22T21:29:45Z

On Wed, 21 Mar 2012 18:52:04 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> As discussed before, I post this to fix the spec and implementation of task moving.
> Then, do you think what target kernel version should be ? 3.4/3.5 ?
> but yes, it may be late for 3.4....

Well, the key information here is "what effect does the bug have upon
users".

> In documentation, it's said that 'shared anon are not moved'.
> But in implementation, the check was wrong.
>
> if (!move_anon() || page_mapcount(page) > 2)
>
> Ah, memcg has been moving shared anon pages for a long time.
>
> Then, here is a discussion about handling of shared anon pages.
>
> - It's complex
> - Now, shared file caches are moved in force.
> - It adds unclear check as page_mapcount(). To do correct check,
> we should check swap users, etc.
> - No one notice this implementation behavior. So, no one get benefit
> from the design.
> - In general, once task is moved to a cgroup for running, it will not
> be moved....
> - Finally, we have control knob as memory.move_charge_at_immigrate.
>
> Here is a patch to allow moving shared pages, completely. This makes
> memcg simpler and fix current broken code.
>
> Note:
> IIUC, libcgroup's cgroup daemon moves tasks after exec().
> So, it's not affected.
> libcgroup's command "cgexec" does move itsef to a memcg and call exec()
> without fork(). it's not affected.
>
> Changelog:
> - fixed PageAnon() check.
> - remove call of lookup_swap_cache()
> - fixed Documentation.

But you forgot to tell us :(