linux-kernel - Re: [RFC 3/3] memcg: get rid of mm

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Message-ID: <20150529152652.GG22728@dhcp22.suse.cz>
Date:	Fri, 29 May 2015 17:26:52 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	Oleg Nesterov <oleg@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 3/3] memcg: get rid of mm_struct::owner

On Fri 29-05-15 11:23:28, Tejun Heo wrote:
> Hello,
> 
> On Fri, May 29, 2015 at 04:57:39PM +0200, Michal Hocko wrote:
[...]
> > OK so you creat a task A (leader) which clones several tasks Pn with
> > CLONE_VM without CLONE_THREAD. Moving A around would control memcg
> > membership while Pn could be moved around freely to control membership
> > in other controllers (e.g. cpu to control shares). So it is something
> > like moving threads separately.
> 
> Sure, it'd behave clearly in certain cases but then again you'd have
> cases where how mm->owner changes isn't clear at all when seen from
> the userland. 

Sure. I am definitely _not_ advocating this use case! As said before, I
consider it abuse. It is just fair to point out this is a user visible
change IMO.

> e.g. When the original owner goes away, the assignment
> of the next owner is essentially arbitrary.  That's what I meant by
> saying it was already a crapshoot.  We should definitely document the
> change but this isn't likely to be an issue.  CLONE_VM &&
> !CLONE_THREAD is an extreme corner case to begin with and even the
> behavior there wasn't all that clearly defined.

That is the line of argumentation in my changelog ;)

-- 
Michal Hocko
SUSE Labs
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