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

From: Michal Hocko <mhocko@kernel.org>
To: Tejun Heo <tj@kernel.org>
Cc: "Roman Gushchin" <guro@fb.com>,
	"Michal Koutný" <mkoutny@suse.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"Andrew Morton" <akpm@linux-foundation.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>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Thu, 14 Nov 2019 20:33:40 +0100	[thread overview]
Message-ID: <20191114193340.GA24848@dhcp22.suse.cz> (raw)
In-Reply-To: <20191114192018.GJ4163745@devbig004.ftw2.facebook.com>

On Thu 14-11-19 11:20:18, Tejun Heo wrote:
> Hello,
> 
> On Thu, Nov 14, 2019 at 08:16:57PM +0100, Michal Hocko wrote:
> > Then what is the point of this function and what about all other users?
> 
> It is useful for controlling admissions of new userspace visible uses
> - e.g. a tracepoint shouldn't be allowed to be attached to a cgroup
> which has already been deleted.

I am not sure I understand. Roman says that the cgroup can get offline
right after the function returns. How is "already deleted" different
from "just deleted"? I thought that the state is preserved at least
while the rcu lock is held but my memory is dim here.

> We're just using it too liberally.

Can we get a doc update to be explicit about sensible usecases so that
others can be dropped accordingly? 
-- 
Michal Hocko
SUSE Labs