Re: [PATCH -v2] memcg: don't do cleanup manually if mem_cgroup_css_online() fails

[Michal Hocko <mhocko@suse.cz>]

On Wed 03-04-13 11:49:29, Li Zefan wrote:
> >> Yes, indeed you are very right - and thanks for looking at such depth.
> > 
> > So what about the patch bellow? It seems that I provoked all this mess
> > but my brain managed to push it away so I do not remember why I thought
> > the parent needs reference drop... It is "only" 3.9 thing fortunately.
> > ---
> >>From 3aff5d958f1d0717795018f7d0d6b63d53ad1dd3 Mon Sep 17 00:00:00 2001
> > From: Li Zefan <lizefan@huawei.com>
> > Date: Tue, 2 Apr 2013 16:37:39 +0200
> > Subject: [PATCH] memcg: don't do cleanup manually if mem_cgroup_css_online()
> >  fails
> > 
> > mem_cgroup_css_online is called with memcg with refcnt = 1 and it
> > expects that mem_cgroup_css_free will drop this last reference.
> > This doesn't hold when memcg_init_kmem fails though and a reference is
> > dropped for both memcg and its parent explicitly if it returns with an
> > error.
> > 
> > This is not correct for two reasons. Firstly mem_cgroup_put on parent is
> > excessive because mem_cgroup_put is hierarchy aware and secondly only
> > memcg_propagate_kmem takes an additional reference.
> > 
> > The first one is a real use-after-free bug introduced by e4715f01
> > (memcg: avoid dangling reference count in creation failure)
> > 
> > The later one is non-issue right now because the only implementation
> > of init_cgroup seems to be tcp_init_cgroup which doesn't fail
> > but it is better to make the error handling saner and move the
> > mem_cgroup_put(memcg) to memcg_propagate_kmem where it belongs.
> > 
> > Signed-off-by: Li Zefan <lizefan@huawei.com>
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
> > ---
> >  mm/memcontrol.c |   13 +++----------
> >  1 file changed, 3 insertions(+), 10 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index f608546..cf9ba7e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5306,6 +5306,8 @@ static int memcg_propagate_kmem(struct mem_cgroup *memcg)
> >  	ret = memcg_update_cache_sizes(memcg);
> >  	mutex_unlock(&set_limit_mutex);
> >  out:
> > +	if (ret)
> > +		mem_cgroup_put(memcg);
> 
> Correct me if I'm wrong, but I think:
> 
> When memcg_propagate_kmem() calls mem_cgroup_get(), it's because the kmemcg
> is active by inheritance. Then when memcg_update_cache_sizes() fails, leading
> to mem_cgroup_css_free() is called by cgroup core:
> 
> static void mem_cgroup_css_free(struct cgroup *cont)
> {
>         struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
> 
>         kmem_cgroup_destroy(memcg);
> 
>         mem_cgroup_put(memcg);
> }
> 
> static void kmem_cgroup_destroy(struct mem_cgroup *memcg)
> {
>         mem_cgroup_sockets_destroy(memcg);
> 
>         memcg_kmem_mark_dead(memcg);
> 
>         if (res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0)
>                 return;
> 
>         if (memcg_kmem_test_and_clear_dead(memcg))
>                 mem_cgroup_put(memcg);    <------- !!!!!!!!!
> }

But memcg_update_cache_sizes calls memcg_kmem_clear_activated on the
error path.

-- 
Michal Hocko
SUSE Labs

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