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

From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: "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>,
	"Tejun Heo" <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Thu, 14 Nov 2019 20:16:57 +0100	[thread overview]
Message-ID: <20191114191657.GN20866@dhcp22.suse.cz> (raw)
In-Reply-To: <20191113170823.GA12464@castle.DHCP.thefacebook.com>

On Wed 13-11-19 17:08:29, Roman Gushchin wrote:
> On Wed, Nov 13, 2019 at 05:29:34PM +0100, Michal Koutný wrote:
> > 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 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. So if we rely somewhere on it, it's already broken.

Then what is the point of this function and what about all other users?

> Generally speaking, it's better to reduce it's usage to the bare minimum.

If it doesn't have any sensible semantic then I would argue it should go
altogether otherwise we are going to chase new users again and aagain?

> > > 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).

The trace however points to current->mm or current->active_memcg. Is it
possible that we have a stale active_memcg?
-- 
Michal Hocko
SUSE Labs