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

[Roman Gushchin]

On Thu, Nov 14, 2019 at 08:16:57PM +0100, Michal Hocko wrote:
> 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@xxxxxx> 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?

That's the plan: to audit all use cases and get rid of it where it's possible.

>  
> > > > 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?

It wouldn't cause a rcu stall.

Thanks!