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

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Date:   Wed, 15 Jul 2020 23:13:22 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Roman Gushchin <guro@...com>
Subject: Re: [External] Re: [PATCH v5.4.y, v4.19.y] mm: memcg/slab: fix memory
 leak at non-root kmem_cache destroy

On Wed, Jul 15, 2020 at 7:32 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 7/7/20 8:27 AM, Muchun Song 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.
> >   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.
> >   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@...edance.com>
>
> CC Roman, who worked in this area recently.
>
> Also why is this marked "[PATCH v5.4.y, v4.19.y]"? Has it been fixed otherwise
> in 5.5+ ?

Because the memcg slab/slub is reworked by Roman since v5.8.
Therefore, this problem exists in v5.7 and below.

>
> > ---
> >  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);
> >
>


--
Yours,
Muchun