linux-kernel - Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from

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Date:   Thu, 14 Nov 2019 20:16:57 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()

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@...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