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

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Message-ID: <20150529140737.GK27479@htj.duckdns.org>
Date:	Fri, 29 May 2015 10:07:37 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Michal Hocko <mhocko@...e.cz>
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

Hello,

On Fri, May 29, 2015 at 03:45:53PM +0200, Michal Hocko wrote:
> Sure but we are talking about processes here. They just happen to share
> mm. And this is exactly the behavior change I am talking about... With

Are we talking about CLONE_VM w/o CLONE_THREAD?  ie. two threadgroups
sharing the same VM?

> the owner you could emulate "threads" with this patch you cannot
> anymore. IMO we shouldn't allow for that but just reading the original
> commit message (cf475ad28ac35) which has added mm->owner:
> "
> It also allows several control groups that are virtually grouped by
> mm_struct, to exist independent of the memory controller i.e., without
> adding mem_cgroup's for each controller, to mm_struct.
> "
> suggests it might have been intentional. That being said, I think it was

I think he's talking about implmenting different controllers which may
want to add their own css pointer in mm_struct now wouldn't need to as
the mm is tagged with the owning task from which membership of all
controllers can be derived.  I don't think that's something we need to
worry about.  We haven't seen even a suggestion for such a controller
and even if that happens we'd be better off adding a separate field
for the new controller.

> a mistake back at the time and we should move on to a saner model. But I
> also believe we should be really vocal when the user visible behavior
> changes. If somebody really asks for the previous behavior I would
> insist on a _strong_ usecase.

I'm a bit lost on what's cleared defined is actually changing.  It's
not like userland had firm control over mm->owner.  It was already a
crapshoot, no?

Thanks.

-- 
tejun
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