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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:     Roman Gushchin <guro@fb.com>, Waiman Long <llong@redhat.com>
Cc:     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>,
        Shakeel Butt <shakeelb@google.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org
References: <20210505154613.17214-1-longman@redhat.com>
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Message-ID: <1b235531-e165-954a-74b1-d3477c2a4b87@redhat.com>
Date:   Wed, 5 May 2021 14:56:55 -0400
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On 5/5/21 2:38 PM, Roman Gushchin wrote:
> On Wed, May 05, 2021 at 02:31:28PM -0400, Waiman Long wrote:
>> On 5/5/21 2:02 PM, Vlastimil Babka wrote:
>>> On 5/5/21 7:30 PM, Roman Gushchin wrote:
>>>> On Wed, May 05, 2021 at 11:46:13AM -0400, Waiman Long wrote:
>>>>> 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.
>>>> Unfortunately the positive effect of this change will be likely
>>>> reversed by a lower utilization due to a larger number of caches.
>>>>
>>>> Btw, I wonder if we also need a change in the slab caches merging procedure?
>>>> KMALLOC_NORMAL caches should not be merged with caches which can potentially
>>>> include accounted objects.
>>> Good point. But looks like kmalloc* caches are extempt from all merging in
>>> create_boot_cache() via
>>>
>>> 	s->refcount = -1;       /* Exempt from merging for now */
>>>
>>> It wouldn't hurt though to create the kmalloc-cg-* caches with SLAB_ACCOUNT flag
>>> to prevent accidental merging in case the above is ever removed. It would also
>>> better reflect reality, and ensure that the array is allocated immediately with
>>> the page, AFAICS.
>>>
>> I am not sure if this is really true.
>>
>> struct kmem_cache *__init create_kmalloc_cache(const char *name,
>>                  unsigned int size, slab_flags_t flags,
>>                  unsigned int useroffset, unsigned int usersize)
>> {
>>          struct kmem_cache *s = kmem_cache_zalloc(kmem_cache, GFP_NOWAIT);
>>
>>          if (!s)
>>                  panic("Out of memory when creating slab %s\n", name);
>>
>>          create_boot_cache(s, name, size, flags, useroffset, usersize);
>>          kasan_cache_create_kmalloc(s);
>>          list_add(&s->list, &slab_caches);
>>          s->refcount = 1;
>>          return s;
>> }
>>
>> Even though refcount is set to -1 initially, it is set back to 1 afterward.
>> So merging can still happen AFAICS.
> Right, thanks, I already noticed it. Then yeah, we should make sure we're not
> merging KMALLOC_NORMAL caches with any others.
>
That should be easy. We just set the refcount to -1 for the 
KMALLOC_NORMAL caches right after its creation then.

Cheers,
Longman