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

From: Michal Koutný
Date: Thu Nov 21 2019 - 10:28:55 EST

On Wed, Nov 13, 2019 at 05:08:29PM +0000, Roman Gushchin <guro@xxxxxx> 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.


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