linux-kernel - Re: [PATCH] memcg: reset to root_mem

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Message-ID: <alpine.LSU.2.00.1112201847500.1310@eggly.anvils>
Date:	Tue, 20 Dec 2011 19:25:04 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cgroups@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH] memcg: reset to root_mem_cgroup at bypassing

On Wed, 21 Dec 2011, KAMEZAWA Hiroyuki wrote:
> On Tue, 20 Dec 2011 09:24:47 +0900
> Hiroyuki Kamezawa <kamezawa.hiroyuki@...il.com> wrote:
> > 2011/12/20 Hugh Dickins <hughd@...gle.com>:
> > 
> > > I speak from experience: I did *exactly* the same at "bypass" when
> > > I introduced our mem_cgroup_reset_page(), which corresponds to your
> > > mem_cgroup_reset_owner(); it seemed right to me that a successful
> > > (return 0) call to try_charge() should provide a good *ptr.
> > >
> > ok.
> > 
> > > But others (Ying and Greg) pointed out that it changes the semantics
> > > of __mem_cgroup_try_charge() in this case, so you need to justify the
> > > change to all those places which do something like "if (ret || !memcg)"
> > > after calling it.  Perhaps it is a good change everywhere, but that's
> > > not obvious, so we chose caution.
> > >
> > > Doesn't it lead to bypass pages being marked as charged to root, so
> > > they don't get charged to the right owner next time they're touched?
> > >
> > Yes. You're right.
> > Hm. So, it seems I should add reset_owner() to the !memcg path
> > rather than here.
> > 
> Considering this again..
> 
> Now, we catch 'charge' event only once in lifetime of anon/file page.
> So, it doesn't depend on that it's marked as PCG_USED or not.

That's an interesting argument, I hadn't been looking at it that way.
It's not true of swapcache, but I guess we don't need to preserve its
peculiarities in this case.

I've not checked the (ret || !memcg) cases yet to see if any change
needed there.

I certainly like that the success return guarantees that memcg is set.

Hugh