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Date:   Tue, 28 Jul 2020 17:27:01 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Sasha Levin <sashal@kernel.org>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Roman Gushchin <guro@fb.com>, Vlastimil Babka <vbabka@suse.cz>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [External] [PATCH 4.19 76/86] mm: memcg/slab: fix memory leak at
 non-root kmem_cache destroy
Message-ID: <20200728152701.GA3554518@kroah.com>
References: <20200727134914.312934924@linuxfoundation.org>
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 <20200728151703.GM406581@sasha-vm>
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On Tue, Jul 28, 2020 at 11:17:03AM -0400, Sasha Levin wrote:
> 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 backported 63b02ef7dc4e ("mm: memcg/slab: synchronize
> access to kmem_cache dying flag using a spinlock") instead of changing
> this patch.

Ah, much better, let me roll my change back and then push out -rc3 with
this all fixed up like this.

thanks,

greg k-h