linux-kernel - Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing in mem_cgroup

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Date:   Thu, 29 Jul 2021 20:12:43 -0700
From:   Roman Gushchin <guro@...com>
To:     Miaohe Lin <linmiaohe@...wei.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <vdavydov.dev@...il.com>, <akpm@...ux-foundation.org>,
        <shakeelb@...gle.com>, <willy@...radead.org>, <alexs@...nel.org>,
        <richard.weiyang@...il.com>, <songmuchun@...edance.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <cgroups@...r.kernel.org>
Subject: Re: [PATCH 4/5] mm, memcg: avoid possible NULL pointer dereferencing
 in mem_cgroup_init()

On Thu, Jul 29, 2021 at 08:57:54PM +0800, Miaohe Lin wrote:
> rtpn might be NULL in very rare case. We have better to check it before
> dereferencing it. Since memcg can live with NULL rb_tree_per_node in
> soft_limit_tree, warn this case and continue.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/memcontrol.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5b4592d1e0f2..70a32174e7c4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7109,6 +7109,8 @@ static int __init mem_cgroup_init(void)
>  		rtpn = kzalloc_node(sizeof(*rtpn), GFP_KERNEL,
>  				    node_online(node) ? node : NUMA_NO_NODE);
>  
> +		if (WARN_ON_ONCE(!rtpn))
> +			continue;

I also really doubt that it makes any sense to continue in this case.
If this allocations fails (at the very beginning of the system's life, it's an __init function),
something is terribly wrong and panic'ing on a NULL-pointer dereference sounds like
a perfect choice.

Is this a real world problem? Do I miss something?