linux-kernel - Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

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Date:   Wed, 5 May 2021 14:31:28 -0400
From:   Waiman Long <llong@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Shakeel Butt <shakeelb@...gle.com>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches

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.

Cheers,
Longman