Re: Accounting problem of MIGRATE_ISOLATED freed page

[Minchan Kim <minchan@kernel.org>]

On 06/23/2012 01:38 PM, Kamezawa Hiroyuki wrote:

> (2012/06/22 15:45), Minchan Kim wrote:
>> On 06/22/2012 10:05 AM, Minchan Kim wrote:
>>
>>> Second approach which is suggested by KOSAKI is what you mentioned.
>>> But the concern about second approach is how to make sure matched
>>> count increase/decrease of nr_isolated_areas.
>>> I mean how to make sure nr_isolated_areas would be zero when
>>> isolation is done.
>>> Of course, we can investigate all of current caller and make sure
>>> they don't make mistake
>>> now. But it's very error-prone if we consider future's user.
>>> So we might need test_set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>>
>>
>> It's an implementation about above approach.
>>
> 
> I like this approach.

> 

> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index bf3404e..3e9a9e1 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -474,6 +474,11 @@ struct zone {
>>           * rarely used fields:
>>           */
>>          const char              *name;
>> +       /*
>> +        * the number of MIGRATE_ISOLATE pageblock
>> +        * We need this for accurate free page counting.
>> +        */
>> +       atomic_t                nr_migrate_isolate;
>>   } ____cacheline_internodealigned_in_smp;
> 
> Isn't this counter modified only under zone->lock ?


AFAIUC, you want to add comment about it. It's no problem. :)

Off-topic:
As I look the code, I found this. Could you confirm this problem?


	CPU A					CPU B

start_isolate_page_range
set_migratetype_isolate
spin_lock_irqsave(zone->lock)
					free_hot_cold_page(Page A)
					migratetype = get_pageblock_migratetype(Page A); /* without zone->lock holding */
					list_add_tail(&page->lru, &pcp->lists[migratetype]); /* Page A could return page into !MIGRATE_ISOLATE */
set_pageblock_migrate
move_freepages_block
drain_all_pages
					/* Page A could be in MIGRATE_MOVABLE of buddy. */
check_pages_isolated
__test_page_isolated_in_pageblock
if (PageBuddy(page A))
	pfn += 1 << page_order(page A);
					/* Page A could be allocated */

__offline_isolated_pages
	BUG_ON(!PageBuddy(page A)); <- HIT! or offline the page is used by someone.

> 
> 
>>
>>   typedef enum {
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2c29b1c..6cb1f9f 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -219,6 +219,11 @@ EXPORT_SYMBOL(nr_online_nodes);
>>
>>   int page_group_by_mobility_disabled __read_mostly;
>>
>> +/*
>> + * NOTE:
>> + * Don't use set_pageblock_migratetype(page, MIGRATE_ISOLATE) direclty.
>> + * Instead, use {un}set_pageblock_isolate.
>> + */
>>   void set_pageblock_migratetype(struct page *page, int migratetype)
>>   {
>>          if (unlikely(page_group_by_mobility_disabled))
>> @@ -1622,6 +1627,28 @@ bool zone_watermark_ok(struct zone *z, int
>> order, unsigned long mark,
>>                                          zone_page_state(z,
>> NR_FREE_PAGES));
>>   }
> 
> I'm glad if this function can be static...Hm. With easy grep, I think it
> can be...


Yes. :)

-- 
Kind regards,
Minchan Kim

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