Re: [External] Re: [PATCH v2] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy

[Roman Gushchin <guro@fb.com>]

On Thu, Jul 16, 2020 at 11:54:37PM +0800, Muchun Song wrote:
> On Thu, Jul 16, 2020 at 11:46 PM Roman Gushchin <guro@fb.com> wrote:
> >
> > On Thu, Jul 16, 2020 at 01:07:02PM +0800, Muchun Song wrote:
> > > On Thu, Jul 16, 2020 at 1:54 AM Roman Gushchin <guro@fb.com> wrote:
> > > >
> > > > On Thu, Jul 16, 2020 at 12:50:22AM +0800, 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
> > > > >      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>
> > > > > Reviewed-by: Shakeel Butt <shakeelb@google.com>
> > > > > ---
> > > > >
> > > > > changelog in v2:
> > > > >  1) Fix a confusing typo in the commit log.
> > > >
> > > > Ok, now I see the problem. Thank you for fixing the commit log!
> > > >
> > > > >  2) Remove flush_memcg_workqueue() for !CONFIG_MEMCG_KMEM.
> > > > >  3) Introduce a new helper memcg_set_kmem_cache_dying() to fix a race
> > > > >     condition between flush_memcg_workqueue() and slab_unmergeable().
> > > > >
> > > > >  mm/slab_common.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++-------
> > > > >  1 file changed, 47 insertions(+), 7 deletions(-)
> > > > >
> > > > > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > > > > index 8c1ffbf7de45..c4958116e3fd 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;
> > > > >  }
> > > > >
> > > > > @@ -886,12 +903,15 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
> > > > >       return 0;
> > > > >  }
> > > > >
> > > > > -static void flush_memcg_workqueue(struct kmem_cache *s)
> > > > > +static void memcg_set_kmem_cache_dying(struct kmem_cache *s)
> > > > >  {
> > > > >       spin_lock_irq(&memcg_kmem_wq_lock);
> > > > >       s->memcg_params.dying = true;
> > > > >       spin_unlock_irq(&memcg_kmem_wq_lock);
> > > > > +}
> > > > >
> > > > > +static void flush_memcg_workqueue(struct kmem_cache *s)
> > > > > +{
> > > > >       /*
> > > > >        * SLAB and SLUB deactivate the kmem_caches through call_rcu. Make
> > > > >        * sure all registered rcu callbacks have been invoked.
> > > > > @@ -923,10 +943,6 @@ static inline int shutdown_memcg_caches(struct kmem_cache *s)
> > > > >  {
> > > > >       return 0;
> > > > >  }
> > > > > -
> > > > > -static inline void flush_memcg_workqueue(struct kmem_cache *s)
> > > > > -{
> > > > > -}
> > > > >  #endif /* CONFIG_MEMCG_KMEM */
> > > > >
> > > > >  void slab_kmem_cache_release(struct kmem_cache *s)
> > > > > @@ -944,8 +960,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > > > >       if (unlikely(!s))
> > > > >               return;
> > > > >
> > > > > -     flush_memcg_workqueue(s);
> > > > > -
> > > > >       get_online_cpus();
> > > > >       get_online_mems();
> > > > >
> > > > > @@ -955,6 +969,32 @@ void kmem_cache_destroy(struct kmem_cache *s)
> > > > >       if (s->refcount)
> > > > >               goto out_unlock;
> > > > >
> > > > > +#ifdef CONFIG_MEMCG_KMEM
> > > > > +     memcg_set_kmem_cache_dying(s);
> > > > > +
> > > > > +     mutex_unlock(&slab_mutex);
> > > >
> > > > Hm, but in theory s->refcount can be increased here?
> > >
> > > I have tried my best to read all the codes that operate on s->refcount.
> > > There is only one place which increases the s->refcount, it is the
> > > __kmem_cache_alias(). If the kmem cache is dying, the slab_unmergeable()
> > > can never return true for the dying kmem cache because it is the same slab_mutex
> > > protection, so I think that there is not a problem, right?
> >
> > So the problem is that you're checking s->refcount under slab_mutex,
> > then you're releasing the mutex and then set the dying bit. But nothing prevents
> 
> Maybe you miss something. The dying bit is set in the
> memcg_set_kmem_cache_dying()
> which is under the slab_mutex protection.  So I think there is no problem.

I'm sorry, I haven't noticed that you've fixed the race in v2.
But then we can probably drop the WARN() and resetting the dying flag part, right?

And because it's a backport-only patch I'd try to make it smaller
(e.g. avoid introducing new helpers, etc) to simplify the back-porting to old kernels.
But it's up to you (and Andrew).

Please, feel free to add
Acked-by: Roman Gushchin <guro@fb.com>
after dropping the dying flag reset part.

Thank you!