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

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Message-ID: <20191113162934.GF19372@blackbody.suse.cz>
Date:   Wed, 13 Nov 2019 17:29:34 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Roman Gushchin <guro@...com>
Cc:     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, kernel-team@...com,
        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()

Hi.

On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin <guro@...com> wrote:
> Let's fix it by switching from css_tryget_online() to css_tryget().
Is this a safe thing to do? The stack captures a kmem charge path, with
css_tryget() it may happen it gets an offlined memcg and carry out
charge into it. What happens when e.g. memcg_deactivate_kmem_caches is
skipped as a consequence?

> The problem is caused by an exiting task which is associated with
> an offline memcg. We're iterating over and over in the
> do {} while (!css_tryget_online()) loop, but obviously the memcg won't
> become online and the exiting task won't be migrated to a live memcg.
As discussed in other replies, the task is not yet exiting. However, the
access to memcg isn't through `current` but `mm->owner`, i.e. another
task of a threadgroup may have got stuck in an offlined memcg (I don't
have a good explanation for that though).

HTH,
Michal

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