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Subject: Re: [PATCH v4 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches
To: Waiman Long <longman@redhat.com>, 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: <20210505200610.13943-1-longman@redhat.com>
 <20210505200610.13943-3-longman@redhat.com>
From: Vlastimil Babka <vbabka@suse.cz>
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Date: Wed, 5 May 2021 23:41:45 +0200
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On 5/5/21 10:06 PM, Waiman Long 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 unaccounted 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 unaccounted
> 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. On the other hand, creating a new set of kmalloc caches does
> have the effect of reducing cache utilization. So it is properly a wash.
> 
> 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>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

A last nitpick: the new caches -cg should perhaps not be created when
cgroup_memory_nokmem == true because kmemcg was disabled by the respective boot
param.