linux-kernel - Re: [PATCH 1/2] mm/memcg: use kmem

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Message-ID: <dp5frcrqofkjjp77hw5sbkri6etnpdsvxnahs6nazvakaxt6im@xouxw25rggci>
Date: Wed, 23 Apr 2025 15:13:47 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Huan Yang <link@...o.com>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH 1/2] mm/memcg: use kmem_cache when alloc memcg

On Wed, Apr 23, 2025 at 02:59:12PM -0700, Andrew Morton wrote:
> On Wed, 23 Apr 2025 16:43:04 +0800 Huan Yang <link@...o.com> wrote:
> 
> > @@ -3652,7 +3654,10 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
> >  	int __maybe_unused i;
> >  	long error;
> >  
> > -	memcg = kzalloc(struct_size(memcg, nodeinfo, nr_node_ids), GFP_KERNEL);
> > +	memcg = likely(memcg_cachep) ?
> > +			kmem_cache_zalloc(memcg_cachep, GFP_KERNEL) :
> > +			kzalloc(struct_size(memcg, nodeinfo, nr_node_ids),
> > +				GFP_KERNEL);
> 
> Why are we testing for memcg_cachep=NULL?
> 
> > @@ -5055,6 +5061,10 @@ static int __init mem_cgroup_init(void)
> >  		INIT_WORK(&per_cpu_ptr(&memcg_stock, cpu)->work,
> >  			  drain_local_stock);
> >  
> > +	memcg_size = struct_size_t(struct mem_cgroup, nodeinfo, nr_node_ids);
> > +	memcg_cachep = kmem_cache_create("mem_cgroup", memcg_size, 0,
> > +					 SLAB_PANIC | SLAB_HWCACHE_ALIGN, NULL);
> 
> If it's because this allocation might have failed then let's not
> bother.  If an __init-time allocation failed, this kernel is unusable
> anyway.

+1 to Andrew's point. SLAB_PANIC is used here, so, memcg_cachep can't be
NULL later.