Re: [RFT PATCH v1 1/5] mm: introduce new field "managed_pages" to struct zone

[Jiang Liu <liuj97@gmail.com>]

On 11/21/2012 03:31 AM, Andrew Morton wrote:

>>>> @@ -106,6 +106,7 @@ static void get_page_bootmem(unsigned long info,  struct page *page,
>>>>  void __ref put_page_bootmem(struct page *page)
>>>>  {
>>>>  	unsigned long type;
>>>> +	static DEFINE_MUTEX(ppb_lock);
>>>>  
>>>>  	type = (unsigned long) page->lru.next;
>>>>  	BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
>>>> @@ -115,7 +116,9 @@ void __ref put_page_bootmem(struct page *page)
>>>>  		ClearPagePrivate(page);
>>>>  		set_page_private(page, 0);
>>>>  		INIT_LIST_HEAD(&page->lru);
>>>> +		mutex_lock(&ppb_lock);
>>>>  		__free_pages_bootmem(page, 0);
>>>> +		mutex_unlock(&ppb_lock);
>>>
>>> The mutex is odd.  Nothing in the changelog, no code comment. 
>>> __free_pages_bootmem() is called from a lot of places but only this one
>>> has locking.  I'm madly guessing that the lock is here to handle two or
>>> more concurrent memory hotpluggings, but I shouldn't need to guess!!
>> Actually I'm a little hesitate whether we should add a lock here.
>>
>> All callers of __free_pages_bootmem() other than put_page_bootmem() should
>> only be used at startup time. And currently the only caller of put_page_bootmem()
>> has already been protected by pgdat_resize_lock(pgdat, &flags). So there's
>> no real need for lock, just defensive.
>>
>> I'm not sure which is the best solution here.
>> 1) add a comments into __free_pages_bootmem() to state that the caller should
>>    serialize themselves.
>> 2) Use a dedicated lock to serialize updates to zone->managed_pages, this need
>>    modifications to page_alloc.c and memory_hotplug.c.
>> 3) The above solution to serialize in put_page_bootmem().
>> What's your suggestions here?
> 
> Firstly, let's be clear about what *data* we're protecting here.  I
> think it's only ->managed_pages?
Yes, we are just trying to protect ->managed_pages.

> I agree that no locking is needed during the init-time code.
> 
> So afaict we only need be concerned about concurrent updates to
> ->managed_pages via memory hotplug, and lock_memory_hotplug() is
> sufficient there.  We don't need to be concerned about readers of
> managed_pages because it is an unsigned long (a u64 on 32-bit machines
> would be a problem).
> 
> All correct?  If so, the code is OK as-is and this can all be
> described/formalised in code comments.  If one wants to be really
> confident, we could do something along the lines of
OK, will add some comments to describe.

> void mod_zone_managed_pages(struct zone *zone, signed long delta)
> {
> 	WARN_ON(system_state != SYSTEM_BOOTING &&
> 		!is_locked_memory_hotplug());
> 	zone->managed_pages += delta;
> }
This seems a little overhead because __free_pages_bootmem() is on the hot path
and will be called many times at boot time.

Regards!
Gerry

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