linux-kernel - Re: [patch 3/8] memcg: update comment about charge reparenting on cgroup exit

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Date:	Mon, 10 Feb 2014 12:12:42 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Johannes Weiner <hannes@...xchg.org>
cc:	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 3/8] memcg: update comment about charge reparenting on
 cgroup exit

On Mon, 10 Feb 2014, Michal Hocko wrote:
> On Fri 07-02-14 12:04:20, Johannes Weiner wrote:
> > Reparenting memory charges in the css_free() callback was meant as a
> > temporary fix for charges that race with offlining, but after some
> > follow-up discussion, it turns out that this is really the right place
> > to reparent charges because it guarantees none are in-flight.

Perhaps: I'm not as gung-ho for this new orthodoxy as you are.

> > 
> > Make clear that the reparenting in css_offline() is an optimistic
> > sweep of established charges because swapout records might hold up
> > css_free() indefinitely, but that in fact the css_free() reparenting
> > is the properly synchronized one.

It worries me that you keep referring to the memsw usage, but
forget the kmem usage, which also delays css_free() indefinitely.

Or am I out-of-date?  Seems not, mem_cgroup_reparent_chages() still
waits for memcg->res - memcg->kmem to reach 0, knowing there's not
much certainty that kmem will reach 0 any time soon.

I think you need a plan for what to do with the kmem pinning,
before going much further in reworking the memsw pinning.

Or at the least, please mention it in this patch's comment.

Hugh

> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> OK, I am still thinking about 2 stage reparenting. LRU drain part called
> from css_offline and charge drain from css_free. But this is a
> sufficient for now.
> 
> Acked-by: Michal Hocko <mhocko@...e.cz>
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