linux-kernel - Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from

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Date:   Thu, 21 Nov 2019 16:28:47 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()

On Wed, Nov 13, 2019 at 05:08:29PM +0000, Roman Gushchin <guro@...com> wrote:
> The thing here is that css_tryget_online() cannot pin the online state,
> so even if returned true, the cgroup can be offline at the return from
> the function.
Adding this for the record.

The actual offlining happens only from css_killed_ref_fn, which is
percpu_ref_kill_and_confirm confirmation callback. That is only called
after RCU grace period. So, css_tryget_online will pin the online state
_inside_ rcu_read_{,un}lock section.

The fact is that many call sites pass the css reference over
rcu_read_unlock boundary allowing potential offlining.

> So if we rely somewhere on it, it's already broken.
Charges into offlined memcg should fine (wrt the original patch). I
didn't check other callers though...

> Generally speaking, it's better to reduce it's usage to the bare minimum.
...I agree as it's confusing and potentially redundant.

Michal

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