Re: [PATCH v3, part4 38/39] mm/hotplug: prepare for removing num_physpages

[Jiang Liu]

On 03/30/2013 12:40 AM, Sam Ravnborg wrote:
> On Sat, Mar 30, 2013 at 12:30:14AM +0800, Jiang Liu wrote:
>> On 03/30/2013 12:17 AM, Sam Ravnborg wrote:
>>>>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>>>>> index 97454b3..9b1b494 100644
>>>>>> --- a/mm/memory_hotplug.c
>>>>>> +++ b/mm/memory_hotplug.c
>>>>>> @@ -751,10 +751,6 @@ EXPORT_SYMBOL_GPL(restore_online_page_callback);
>>>>>>  
>>>>>>  void __online_page_set_limits(struct page *page)
>>>>>>  {
>>>>>> -	unsigned long pfn = page_to_pfn(page);
>>>>>> -
>>>>>> -	if (pfn >= num_physpages)
>>>>>> -		num_physpages = pfn + 1;
>>>>>>  }
>>>>>>  EXPORT_SYMBOL_GPL(__online_page_set_limits);
>>>>>
>>>>> How can this be correct?
>>>>> With this change __online_page_set_limits() is now a nop.
>>>> Hi Sam,
>>>> 	We will eventually remove the global variable num_physpages in the last patch.
>>>> I kept the nop __online_page_set_limits() because I have a plan to use it to fix other
>>>> bugs in memory hotplug, otherwise it may be killed too.
>>>
>>> The xen ballon driver uses __online_page_set_limits for memory
>>> hotplug - so this will break this driver afaics.
>> Hi Sam,
>> 	I haven't gotten your point yet here. 
>> 	Function __online_page_set_limits() was only used to update the global variable
>> num_physpages, and one of the goals of this patch set is to get rid of num_physpages.
>> So I think it won't break Xen balloon driver.
>> 	Please refer to the patch here, which eventually kills num_physpages.
>> http://marc.info/?l=linux-mm&m=136431387813309&w=2
> 
> I assumed that num_physpages was used run-time to check for the
> available number of pages. And __online_page_set_limits() were used
> to reduced the number of online pages.
> 
> But a quick grep did not reveal any such use.
> 
> So it looks OK - thanks for the clarification.
Yeah, we have a patch to replace num_physpages with totalram_pages for runtime access,
please refer to:
http://marc.info/?l=linux-mm&m=136431351413052&w=2

> 
> 	Sam
> 

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