linux-kernel - Re: [RFC][-mm] Memory controller add mm->owner

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Date:	Mon, 24 Mar 2008 09:34:39 -0700
From:	"Paul Menage" <menage@...gle.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	linux-mm@...ck.org, "Hugh Dickins" <hugh@...itas.com>,
	"Sudhir Kumar" <skumar@...ux.vnet.ibm.com>,
	"YAMAMOTO Takashi" <yamamoto@...inux.co.jp>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, taka@...inux.co.jp,
	"David Rientjes" <rientjes@...gle.com>,
	"Pavel Emelianov" <xemul@...nvz.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC][-mm] Memory controller add mm->owner

On Mon, Mar 24, 2008 at 9:21 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>  > Also, if mm->owner exits but mm is still alive (unlikely, but could
>  > happen with weird custom threading libraries?) then we need to
>  > reassign mm->owner to one of the other users of the mm (by looking
>  > first in the thread group, then among the parents/siblings/children,
>  > and then among all processes as a last resort?)
>  >
>
>  The comment in __exit_signal states that
>
>  "The group leader stays around as a zombie as long
>   as there are other threads.  When it gets reaped,
>   the exit.c code will add its counts into these totals."

Ah, that's useful to know.

>
>  Given that the thread group leader stays around, do we need to reassign
>  mm->owner? Do you do anything special in cgroups like cleanup the
>  task_struct->css->subsys_state on exit?
>

OK, so we don't need to handle this for NPTL apps - but for anything
still using LinuxThreads or manually constructed clone() calls that
use CLONE_VM without CLONE_PID, this could still be an issue. (Also I
guess there's the case of someone holding a reference to the mm via a
/proc file?)

>
>  >>  -       rcu_read_lock();
>  >>  -       mem = rcu_dereference(mm->mem_cgroup);
>  >>  +       mem = mem_cgroup_from_task(mm->owner);
>  >
>  > I think we still need the rcu_read_lock(), since mm->owner can move
>  > cgroups any time.
>  >
>
>  OK, so cgroup task movement is protected by RCU, right? I'll check for all
>  mm->owner uses.
>

Yes - cgroup_attach() uses synchronize_rcu() before release the cgroup
mutex. So although you can't guarantee that the cgroup set won't
change if you're just using RCU, you can't guarantee that you're
addressing a still-valid non-destroyed (and of course non-freed)
cgroup set.

Paul
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