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

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Date:	Tue, 11 Feb 2014 13:48:43 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Hugh Dickins <hughd@...gle.com>
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, Feb 10, 2014 at 12:12:42PM -0800, Hugh Dickins wrote:
> 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 Or am I out-of-date?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.

It think the discussion from the other thread bled over into this one
a little bit, this patch was merely about clarifying that .css_free()
reparenting is not the crude hack it was described as.

Yes, I forgot about kmem and it should be mentioned in this patch.
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