Re: [PATCH v2] zswap: only use CPU notifier when HOTPLUG_CPU=y

[Dan Streetman <ddstreet@ieee.org>]

On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@kernel.org> 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?

hmm, that's interesting, __unregister_cpu_notifier is always a noop if
HOTPLUG_CPU=n, but __register_cpu_notifier is only a noop if
HOTPLUG_CPU=n *and* MODULE.  If !MODULE, __register_cpu_notifier does
actually register!  This was added by commit
47e627bc8c9a70392d2049e6af5bd55fae61fe53 ('hotplug: Allow modules to
use the cpu hotplug notifiers even if !CONFIG_HOTPLUG_CPU') and looks
like it's to allow built-ins to register so they can notice during
boot when cpus are initialized.

IMHO, that is the real problem - sure, without HOTPLUG_CPU, nobody
should ever get a notification that a cpu is dying, but that doesn't
mean builtins that register notifiers will never unregister their
notifiers and then free them.

Changing zswap is only working around the symptom; instead, hotplug
should be changed to provide unregister_cpu_notifier in all cases
where register_cpu_notifier is provided.

>
>> Signe-off-by: Yu Zhao <yuzhao@google.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
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@kvack.org.  For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
>
> --
> Michal Hocko
> SUSE Labs