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

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Message-ID: <3txgkddzf62xhxwlzm63ip3tqv3r2tmd4elnka2z5ya7ngqr62@f554paqdco5s>
Date: Wed, 23 Apr 2025 20:31:17 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Huan Yang <link@...o.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	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 Thu, Apr 24, 2025 at 10:45:05AM +0800, Huan Yang wrote:
> Hi Shakeel
> 
> 在 2025/4/24 06:13, Shakeel Butt 写道:
> > 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.
> 
> I see cgroup_init(in start_kernel) ahead of initcall( which in rest_init->kernel_init->...->initcall). So, root_mem_cgroup create use
> 
> cachep will trigger NULL pointer access.

So, either create a new function which creates this kmem cache before
cgroup_init() or just call mem_cgroup_init() before cgroup_init()
(similar to cpuset_init()).