Re: [External] Re: [PATCH v5.4.y, v4.19.y] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Jul 15, 2020 at 11:21 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> Sorry I missed this email.
>
>
> On Mon, Jul 6, 2020 at 11:28 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > If the kmem_cache refcount is greater than one, we should not
> > mark the root kmem_cache as dying. If we mark the root kmem_cache
> > dying incorrectly, the non-root kmem_cache can never be destroyed.
> > It resulted in memory leak when memcg was destroyed. We can use the
> > following steps to reproduce.
> >
> >   1) Use kmem_cache_create() to create a new kmem_cache named A.
> >   2) Coincidentally, the kmem_cache A is an alias for kmem_cache B,
> >      so the refcount of B is just increased.
>
> I definitely missed the alias kmem cache case.
>
> >   3) Use kmem_cache_destroy() to destroy the kmem_cache A, just
> >      decrease the B's refcount but mark the B as dying.
> >   4) Create a new memory cgroup and alloc memory from the kmem_cache
> >      A. It leads to create a non-root kmem_cache for allocating.
>
> I think in (4) you meant alloc memory from kmem_cache B instead of A.
> There should not be any allocation from A after kmem_cache_destroy()
> in (3).

Yeah, here is a typo. I will fix it in the next version patch.

>
> >   5) When destroy the memory cgroup created in the step 4), the
> >      non-root kmem_cache can never be destroyed.
> >
> > If we repeat steps 4) and 5), this will cause a lot of memory leak.
> > So only when refcount reach zero, we mark the root kmem_cache as dying.
> >
> > Fixes: 92ee383f6daa ("mm: fix race between kmem_cache destroy, create and deactivate")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> The patch looks fine.
>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>
> > ---
> >  mm/slab_common.c | 43 +++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 41 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 8c1ffbf7de45..83ee6211aec7 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -258,6 +258,11 @@ static void memcg_unlink_cache(struct kmem_cache *s)
> >                 list_del(&s->memcg_params.kmem_caches_node);
> >         }
> >  }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > +       return is_root_cache(s) && s->memcg_params.dying;
> > +}
> >  #else
> >  static inline int init_memcg_params(struct kmem_cache *s,
> >                                     struct kmem_cache *root_cache)
> > @@ -272,6 +277,11 @@ static inline void destroy_memcg_params(struct kmem_cache *s)
> >  static inline void memcg_unlink_cache(struct kmem_cache *s)
> >  {
> >  }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > +       return false;
> > +}
> >  #endif /* CONFIG_MEMCG_KMEM */
> >
> >  /*
> > @@ -326,6 +336,13 @@ int slab_unmergeable(struct kmem_cache *s)
> >         if (s->refcount < 0)
> >                 return 1;
> >
> > +       /*
> > +        * If the kmem_cache is dying. We should also skip this
> > +        * kmem_cache.
> > +        */
> > +       if (memcg_kmem_cache_dying(s))
> > +               return 1;
> > +
> >         return 0;
> >  }
> >
> > @@ -944,8 +961,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >         if (unlikely(!s))
> >                 return;
> >
> > -       flush_memcg_workqueue(s);
> > -
> >         get_online_cpus();
> >         get_online_mems();
> >
> > @@ -955,6 +970,30 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >         if (s->refcount)
> >                 goto out_unlock;
> >
> > +#ifdef CONFIG_MEMCG_KMEM
> > +       mutex_unlock(&slab_mutex);
> > +
> > +       put_online_mems();
> > +       put_online_cpus();
> > +
> > +       flush_memcg_workqueue(s);
> > +
> > +       get_online_cpus();
> > +       get_online_mems();
> > +
> > +       mutex_lock(&slab_mutex);
> > +
> > +       if (WARN(s->refcount,
> > +                "kmem_cache_destroy %s: Slab cache is still referenced\n",
> > +                s->name)) {
> > +               /*
> > +                * Reset the dying flag setted by flush_memcg_workqueue().
> > +                */
> > +               s->memcg_params.dying = false;
> > +               goto out_unlock;
> > +       }
> > +#endif
> > +
> >         err = shutdown_memcg_caches(s);
> >         if (!err)
> >                 err = shutdown_cache(s);
> > --
> > 2.11.0
> >



-- 
Yours,
Muchun