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

From: "Michal Koutný" <mkoutny@suse.com>
To: Roman Gushchin <guro@fb.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Thu, 21 Nov 2019 16:28:47 +0100	[thread overview]
Message-ID: <20191121152847.GA14375@blackbody.suse.cz> (raw)
In-Reply-To: <20191113170823.GA12464@castle.DHCP.thefacebook.com>

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