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

[Sasha Levin]

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@xxxxxxxxxxxxxxxxxxx> wrote:
> > From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Acked-by: Roman Gushchin <guro@xxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Link: http://lkml.kernel.org/r/20200716165103.83462-1-songmuchun@xxxxxxxxxxxxx
> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >
> > ---
> >  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 backported 63b02ef7dc4e ("mm: memcg/slab: synchronize
access to kmem_cache dying flag using a spinlock") instead of changing
this patch.

--
Thanks,
Sasha