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Date:   Mon, 18 Apr 2022 20:55:31 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     <vbabka@suse.cz>, <pintu@codeaurora.org>,
        <charante@codeaurora.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH 09/12] mm: compaction: avoid possible NULL pointer
 dereference in kcompactd_cpu_online
Message-Id: <20220418205531.2e4ed0f72e0e537ef55c6468@linux-foundation.org>
In-Reply-To: <20220418141253.24298-10-linmiaohe@huawei.com>
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        <20220418141253.24298-10-linmiaohe@huawei.com>
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Please cc David H on memhotplug stuff.

On Mon, 18 Apr 2022 22:12:50 +0800 Miaohe Lin <linmiaohe@huawei.com> wrote:

> It's possible that kcompactd_run could fail to run kcompactd for a hot
> added node and leave pgdat->kcompactd as NULL. So pgdat->kcompactd should
> be checked here to avoid possible NULL pointer dereference.
>
> ..
>
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -3052,7 +3052,8 @@ static int kcompactd_cpu_online(unsigned int cpu)
>  
>  		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
>  			/* One of our CPUs online: restore mask */
> -			set_cpus_allowed_ptr(pgdat->kcompactd, mask);
> +			if (pgdat->kcompactd)
> +				set_cpus_allowed_ptr(pgdat->kcompactd, mask);
>  	}
>  	return 0;
>  }

Why not fail to bring the node online if kcompactd_run() failed?

Also, should we panic the system if kcompactd_run() failed in
kcompactd_init()?