Re: [RFC PATCH] memory-hotplug: Add memblock_state notifier

[Wen Congyang <wencongyang@gmail.com>]

At 2012/7/23 19:06, Vasilis Liaskovitis Wrote:
> Hi,
>
> On Mon, Jul 23, 2012 at 05:08:04PM +0800, Wen Congyang wrote:
>>> +static int memblock_state_notifier_nb(struct notifier_block *nb, unsigned long
>>> +		val, void *v)
>>> +{
>>> +	struct memory_notify *arg = (struct memory_notify *)v;
>>> +	struct memory_block *mem = NULL;
>>> +	struct mem_section *ms;
>>> +	unsigned long section_nr;
>>> +
>>> +	section_nr = pfn_to_section_nr(arg->start_pfn);
>>> +	ms = __nr_to_section(section_nr);
>>> +	mem = find_memory_block(ms);
>>> +	if (!mem)
>>> +		goto out;
>>
>> we may offline more than one memory block.
>>
> thanks, you are right.
>
>>> +
>>> +	switch (val) {
>>> +	case MEM_GOING_OFFLINE:
>>> +	case MEM_OFFLINE:
>>> +	case MEM_GOING_ONLINE:
>>> +	case MEM_ONLINE:
>>> +	case MEM_CANCEL_ONLINE:
>>> +	case MEM_CANCEL_OFFLINE:
>>> +		mem->state = val;
>>
>> mem->state is protected by the lock mem->state_mutex, so if you want to
>> update the state, you must lock mem->state_mutex. But you cannot lock it
>> here, because it may cause deadlock:
>>
>> acpi_memhotplug                           sysfs interface
>> ===============================================================================
>>                                            memory_block_change_state()
>>                                                lock mem->state_mutex
>>                                                memory_block_action()
>> offline_pages()
>>      lock_memory_hotplug()
>>                                                    offline_memory()
>>                                                        lock_memory_hotplug() // block
>>      memory_notify()
>>          memblock_state_notifier_nb()
>> ===============================================================================
>
> good point. Maybe if memory_hotplug_lock and state_mutex locks are acquired in
> the same order in the 2 code paths, this could be avoided.

Yes, I am trying to fix another 2 problems(also based on ishimatsu's 
patchset):
1. offline_memory() will fail if part of the memory is onlined and part 
of the memory
    is offlined.
2. notify the userspace if the memory block's status is changed

I guess this problem can be fixed together.

Thanks
Wen Congyang

>
>> I'm writing another patch to fix it.
>
> ok, I 'll test.
> thanks,
>
> - Vasilis
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