linux-kernel - Re: [PATCH 2/2] cgroup: revert fa06235b8eb0 ("cgroup: reset css on destruction")

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Message-ID: <20170727143617.GC19738@cmpxchg.org>
Date:   Thu, 27 Jul 2017 10:36:17 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-kernel@...r.kernel.org,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...nel.org>,
        kernel-team@...com, cgroups@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] cgroup: revert fa06235b8eb0 ("cgroup: reset css on
 destruction")

On Thu, Jul 27, 2017 at 02:04:28PM +0100, Roman Gushchin wrote:
> Commit fa06235b8eb0 ("cgroup: reset css on destruction") caused
> css_reset callback to be called from the offlining path. Although
> it solves the problem mentioned in the commit description
> ("For instance, memory cgroup needs to reset memory.low, otherwise
> pages charged to a dead cgroup might never get reclaimed."),
> generally speaking, it's not correct.
> 
> An offline cgroup can still be a resource domain, and we shouldn't
> grant it more resources than it had before deletion.
> 
> For instance, if an offline memory cgroup has dirty pages, we should
> still imply i/o limits during writeback.
> 
> The css_reset callback is designed to return the cgroup state
> into the original state, that means reset all limits and counters.
> It's spomething different from the offlining, and we shouldn't use
> it from the offlining path. Instead, we should adjust necessary
> settings from the per-controller css_offline callbacks (e.g. reset
> memory.low).
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: kernel-team@...com
> Cc: cgroups@...r.kernel.org
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org

Acked-by: Johannes Weiner <hannes@...xchg.org>