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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches
To:     Shakeel Butt <shakeelb@google.com>
Cc:     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>,
        Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>
References: <20210505154613.17214-1-longman@redhat.com>
 <20210505154613.17214-3-longman@redhat.com>
 <CALvZod7TzBVdwdCMChFNEZqYHxQUWBVfvWwtuAH-4rh_b4XRKw@mail.gmail.com>
Message-ID: <dbeb319b-81bb-ac4f-25f4-dd275834cd98@redhat.com>
Date:   Wed, 5 May 2021 12:31:57 -0400
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On 5/5/21 12:17 PM, Shakeel Butt wrote:
> On Wed, May 5, 2021 at 8:47 AM Waiman Long <longman@redhat.com> wrote:
>> There are currently two problems in the way the objcg pointer array
>> (memcg_data) in the page structure is being allocated and freed.
>>
>> On its allocation, it is possible that the allocated objcg pointer
>> 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.
>>
>> When it is freed, the objcg pointer array object may be the last one
>> in its slab and hence causes kfree() to be called again. With the
>> right workload, the slab cache may be set up in a way that allows the
>> recursive kfree() calling loop to nest deep enough to cause a kernel
>> stack overflow and panic the system.
>>
>> One way to solve this problem is to split the kmalloc-<n> caches
>> (KMALLOC_NORMAL) into two separate sets - a new set of kmalloc-<n>
>> (KMALLOC_NORMAL) caches for non-accounted objects only and a new set of
>> kmalloc-cg-<n> (KMALLOC_CGROUP) caches for accounted objects only. All
>> the other caches can still allow a mix of accounted and non-accounted
>> objects.
>>
>> With this change, all the objcg pointer array objects will come from
>> KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So
>> both the recursive kfree() problem and non-freeable slab problem are
>> gone. Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer
>> have mixed accounted and unaccounted objects, this will slightly reduce
>> the number of objcg pointer arrays that need to be allocated and save
>> a bit of memory.
>>
>> The new KMALLOC_CGROUP is added between KMALLOC_NORMAL and
>> KMALLOC_RECLAIM so that the first for loop in create_kmalloc_caches()
>> will include the newly added caches without change.
>>
>> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> One nit below and after incorporating Vlastimil's suggestions:
>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
>
>> ---
>>   include/linux/slab.h | 42 ++++++++++++++++++++++++++++++++++--------
>>   mm/slab_common.c     | 23 +++++++++++++++--------
>>   2 files changed, 49 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 0c97d788762c..f2d9ebc34f5c 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -305,9 +305,16 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>>   /*
>>    * Whenever changing this, take care of that kmalloc_type() and
>>    * create_kmalloc_caches() still work as intended.
>> + *
>> + * KMALLOC_NORMAL is for non-accounted objects only whereas KMALLOC_CGROUP
>> + * is for accounted objects only.
> I think you can say "KMALLOC_CGROUP is for accounted and unreclaimable
> objects only".
>
Thanks for the suggestion. Will incorporate that.

Cheers,
Longman