On Fri, Dec 2, 2016 at 8:46 AM, Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>> 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
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