Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Wed, Aug 01, 2018 at 11:55:52AM -0400, Johannes Weiner wrote:
> On Tue, Jul 31, 2018 at 04:39:08PM -0700, Andrew Morton wrote:
> > On Mon, 30 Jul 2018 11:31:13 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> > 
> > > Subject: [PATCH] mm: memcontrol: simplify memcg idr allocation and error
> > >  unwinding
> > > 
> > > The memcg ID is allocated early in the multi-step memcg creation
> > > process, which needs 2-step ID allocation and IDR publishing, as well
> > > as two separate IDR cleanup/unwind sites on error.
> > > 
> > > Defer the IDR allocation until the last second during onlining to
> > > eliminate all this complexity. There is no requirement to have the ID
> > > and IDR entry earlier than that. And the root reference to the ID is
> > > put in the offline path, so this matches nicely.
> > 
> > This patch isn't aware of Kirill's later "mm, memcg: assign memcg-aware
> > shrinkers bitmap to memcg", which altered mem_cgroup_css_online():
> > 
> > @@ -4356,6 +4470,11 @@ static int mem_cgroup_css_online(struct
> >  {
> >  	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> >  
> > +	if (memcg_alloc_shrinker_maps(memcg)) {
> > +		mem_cgroup_id_remove(memcg);
> > +		return -ENOMEM;
> > +	}
> > +
> >  	/* Online state pins memcg ID, memcg ID pins CSS */
> >  	atomic_set(&memcg->id.ref, 1);
> >  	css_get(css);
> > 
> 
> Hm, that looks out of place too. The bitmaps are allocated for the
> entire lifetime of the css, not just while it's online.
> 
> Any objections to the following fixup to that patch?

That would be incorrect. Memory cgroups that haven't been put online
are invisible to for_each_mem_cgroup(), which is used for expanding
shrinker maps of all cgroups - see memcg_expand_shrinker_maps(). So if
memcg_expand_shrinker_maps() is called between css_alloc and css_online,
it will miss this cgroup and its shrinker_map won't be reallocated to
fit the new id. Allocating the shrinker map in css_online guarantees
that it won't happen. Looks like this code lacks a comment...