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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab
To:     Vlastimil Babka <vbabka@suse.cz>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>
Cc:     linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org
References: <20210502180755.445-1-longman@redhat.com>
 <20210502180755.445-2-longman@redhat.com>
 <699e5ac8-9044-d664-f73f-778fe72fd09b@suse.cz>
Message-ID: <4c90cf79-9c61-8964-a6fd-2da087893339@redhat.com>
Date:   Mon, 3 May 2021 10:20:17 -0400
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On 5/3/21 8:22 AM, Vlastimil Babka wrote:
> On 5/2/21 8:07 PM, Waiman Long wrote:
>> The obj_cgroup array (memcg_data) embedded in the page structure is
>> allocated at the first instance an accounted memory allocation happens.
>> With the right size object, it is possible that the allocated obj_cgroup
>> array comes from the same slab that requires memory accounting. If this
>> happens, the slab will never become empty again as there is at least one
>> object left (the obj_cgroup array) in the slab.
>>
>> With instructmentation code added to detect this situation, I got 76
>> hits on the kmalloc-192 slab when booting up a test kernel on a VM.
>> So this can really happen.
>>
>> To avoid the creation of these unfreeable slabs, a check is added to
>> memcg_alloc_page_obj_cgroups() to detect that and double the size
>> of the array in case it happens to make sure that it comes from a
>> different kmemcache.
>>
>> This change, however, does not completely eliminate the presence
>> of unfreeable slabs which can still happen if a circular obj_cgroup
>> array dependency is formed.
> Hm this looks like only a half fix then.
> I'm afraid the proper fix is for kmemcg to create own set of caches for the
> arrays. It would also solve the recursive kfree() issue.

Right, this is a possible solution. However, the objcg pointers array 
should need that much memory. Creating its own set of kmemcaches may 
seem like an overkill.

Cheers,
Longman