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Date:   Tue, 28 Jul 2020 11:17:03 -0400
From:   Sasha Levin <sashal@kernel.org>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        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: <20200728151703.GM406581@sasha-vm>
References: <20200727134914.312934924@linuxfoundation.org>
 <20200727134918.205538211@linuxfoundation.org>
 <CAMZfGtWVtGeMfu=04LiNVcLrBpmexUryHjy-dujo77CpJhcwGg@mail.gmail.com>
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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.

-- 
Thanks,
Sasha