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

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Date:   Mon, 5 Dec 2016 13:11:26 -0800
From:   Yu Zhao <yuzhao@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
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 Fri, Dec 02, 2016 at 02:46:06PM +0100, Michal Hocko wrote:
> 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?

Well, I'm not a fan of ifdef and I don't like the unnecessary memory
usage (notifier_block) and lock (cpu_notifier_register_begin/done)
either.

Just pointing this out, having no problem living with your hotplug
fixes.

> 
> > 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