linux-kernel - Re: [PATCH v2] zswap: only use CPU notifier when HOTPLUG

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Date:   Fri, 2 Dec 2016 14:46:06 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zswap: only use CPU notifier when HOTPLUG_CPU=y

On Wed 30-11-16 13:15:16, Yu Zhao wrote:
> __unregister_cpu_notifier() only removes registered notifier from its
> linked list when CPU hotplug is configured. If we free registered CPU
> notifier when HOTPLUG_CPU=n, we corrupt the linked list.
> 
> To fix the problem, we can either use a static CPU notifier that walks
> through each pool or just simply disable CPU notifier when CPU hotplug
> is not configured (which is perfectly safe because the code in question
> is called after all possible CPUs are online and will remain online
> until power off).
> 
> v2: #ifdef for cpu_notifier_register_done during cleanup.

this ifedfery is just ugly as hell. I am also wondering whether it is
really needed. __register_cpu_notifier and __unregister_cpu_notifier are
noops for CONFIG_HOTPLUG_CPU=n. So what's exactly that is broken here?

> Signe-off-by: Yu Zhao <yuzhao@...gle.com>
> ---
>  mm/zswap.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 275b22c..2915f44 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -118,7 +118,9 @@ struct zswap_pool {
>  	struct kref kref;
>  	struct list_head list;
>  	struct work_struct work;
> +#ifdef CONFIG_HOTPLUG_CPU
>  	struct notifier_block notifier;
> +#endif
>  	char tfm_name[CRYPTO_MAX_ALG_NAME];
>  };
>  
> @@ -448,6 +450,7 @@ static int __zswap_cpu_comp_notifier(struct zswap_pool *pool,
>  	return NOTIFY_OK;
>  }
>  
> +#ifdef CONFIG_HOTPLUG_CPU
>  static int zswap_cpu_comp_notifier(struct notifier_block *nb,
>  				   unsigned long action, void *pcpu)
>  {
> @@ -456,27 +459,34 @@ static int zswap_cpu_comp_notifier(struct notifier_block *nb,
>  
>  	return __zswap_cpu_comp_notifier(pool, action, cpu);
>  }
> +#endif
>  
>  static int zswap_cpu_comp_init(struct zswap_pool *pool)
>  {
>  	unsigned long cpu;
>  
> +#ifdef CONFIG_HOTPLUG_CPU
>  	memset(&pool->notifier, 0, sizeof(pool->notifier));
>  	pool->notifier.notifier_call = zswap_cpu_comp_notifier;
>  
>  	cpu_notifier_register_begin();
> +#endif
>  	for_each_online_cpu(cpu)
>  		if (__zswap_cpu_comp_notifier(pool, CPU_UP_PREPARE, cpu) ==
>  		    NOTIFY_BAD)
>  			goto cleanup;
> +#ifdef CONFIG_HOTPLUG_CPU
>  	__register_cpu_notifier(&pool->notifier);
>  	cpu_notifier_register_done();
> +#endif
>  	return 0;
>  
>  cleanup:
>  	for_each_online_cpu(cpu)
>  		__zswap_cpu_comp_notifier(pool, CPU_UP_CANCELED, cpu);
> +#ifdef CONFIG_HOTPLUG_CPU
>  	cpu_notifier_register_done();
> +#endif
>  	return -ENOMEM;
>  }
>  
> @@ -484,11 +494,15 @@ static void zswap_cpu_comp_destroy(struct zswap_pool *pool)
>  {
>  	unsigned long cpu;
>  
> +#ifdef CONFIG_HOTPLUG_CPU
>  	cpu_notifier_register_begin();
> +#endif
>  	for_each_online_cpu(cpu)
>  		__zswap_cpu_comp_notifier(pool, CPU_UP_CANCELED, cpu);
> +#ifdef CONFIG_HOTPLUG_CPU
>  	__unregister_cpu_notifier(&pool->notifier);
>  	cpu_notifier_register_done();
> +#endif
>  }
>  
>  /*********************************
> -- 
> 2.8.0.rc3.226.g39d4020
> 
> --
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-- 
Michal Hocko
SUSE Labs