Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

Michal Koutný
2019-11-13T16:29:39Z

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Hi.

On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin <guro@fb.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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