Re: [External] [PATCH 4.19 76/86] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy

Greg Kroah-Hartman
2020-07-28T15:00:55Z

On Tue, Jul 28, 2020 at 08:56:41PM +0800, Muchun Song wrote:
> On Mon, Jul 27, 2020 at 10:12 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > From: Muchun Song <songmuchun@bytedance.com>
> >
> > commit d38a2b7a9c939e6d7329ab92b96559ccebf7b135 upstream.
> >
> > 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
> > B. It leads to create a non-root kmem_cache for allocating memory.
> > 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>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > Reviewed-by: Shakeel Butt <shakeelb@google.com>
> > Acked-by: Roman Gushchin <guro@fb.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: <stable@vger.kernel.org>
> > Link: http://lkml.kernel.org/r/20200716165103.83462-1-songmuchun@bytedance.com
> > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >
> > ---
> > mm/slab_common.c | 35 ++++++++++++++++++++++++++++-------
> > 1 file changed, 28 insertions(+), 7 deletions(-)
> >
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -310,6 +310,14 @@ int slab_unmergeable(struct kmem_cache *
> > if (s->refcount < 0)
> > return 1;
> >
> > +#ifdef CONFIG_MEMCG_KMEM
> > + /*
> > + * Skip the dying kmem_cache.
> > + */
> > + if (s->memcg_params.dying)
> > + return 1;
> > +#endif
> > +
> > return 0;
> > }
> >
> > @@ -832,12 +840,15 @@ static int shutdown_memcg_caches(struct
> > return 0;
> > }
> >
> > -static void flush_memcg_workqueue(struct kmem_cache *s)
> > +static void memcg_set_kmem_cache_dying(struct kmem_cache *s)
> > {
> > mutex_lock(&slab_mutex);
> > s->memcg_params.dying = true;
> > mutex_unlock(&slab_mutex);
>
> We should remove mutex_lock/unlock(&slab_mutex) here, because
> we already hold the slab_mutex from kmem_cache_destroy().

Good catch. I'll go make this change and push out a -rc2.

thanks,

greg k-h