Re: [Patch v4 3/8] memory-hotplug: fix NR_FREE_PAGES mismatch

[Wen Congyang <wency@cn.fujitsu.com>]

At 10/31/2012 09:41 PM, Jianguo Wu Wrote:
> On 2012/10/31 19:23, Wen Congyang wrote:
>> NR_FREE_PAGES will be wrong after offlining pages.  We add/dec
>> NR_FREE_PAGES like this now:
>>
>> 1. move all pages in buddy system to MIGRATE_ISOLATE, and dec NR_FREE_PAGES
>>
>> 2. don't add NR_FREE_PAGES when it is freed and the migratetype is
>>    MIGRATE_ISOLATE
>>
>> 3. dec NR_FREE_PAGES when offlining isolated pages.
>>
>> 4. add NR_FREE_PAGES when undoing isolate pages.
>>
>> When we come to step 3, all pages are in MIGRATE_ISOLATE list, and
>> NR_FREE_PAGES are right.  When we come to step4, all pages are not in
>> buddy system, so we don't change NR_FREE_PAGES in this step, but we change
>> NR_FREE_PAGES in step3.  So NR_FREE_PAGES is wrong after offlining pages.
>> So there is no need to change NR_FREE_PAGES in step3.
>>
>> This patch also fixs a problem in step2: if the migratetype is
>> MIGRATE_ISOLATE, we should not add NR_FRR_PAGES when we remove pages from
>> pcppages.
>>
>> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
>> Cc: David Rientjes <rientjes@google.com>
>> Cc: Jiang Liu <liuj97@gmail.com>
>> Cc: Len Brown <len.brown@intel.com>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Minchan Kim <minchan.kim@gmail.com>
>> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
>> Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>> Cc: Dave Hansen <dave@linux.vnet.ibm.com>
>> Cc: Mel Gorman <mel@csn.ul.ie>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>  mm/page_alloc.c | 10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 5b74de6..a7cd2d1 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -667,11 +667,13 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>>  			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
>>  			__free_one_page(page, zone, 0, mt);
>>  			trace_mm_page_pcpu_drain(page, 0, mt);
>> -			if (is_migrate_cma(mt))
>> -				__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
>> +			if (likely(mt != MIGRATE_ISOLATE)) {
> 
> Hi Congyang,
> 	I think mt != MIGRATE_ISOLATE is always true here,
> page from PCP's migratetype < MIGRATE_PCPTYPES.
> When isolate page, we change pageblock's migratetype to MIGRATE_ISOLATE,
> but set_freepage_migratetype() isn't called.
> Maybe we can use mt = get_pageblock_migratetype() here ?

Yes, you are right. I have sent a fix patch.

Thanks for pointing it out.

Wen Congyang

> 
> Thanks,
> Jianguo Wu.
> 
>> +				__mod_zone_page_state(zone, NR_FREE_PAGES, 1);
>> +				if (is_migrate_cma(mt))
>> +					__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, 1);
>> +			}
>>  		} while (--to_free && --batch_free && !list_empty(list));
>>  	}
>> -	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
>>  	spin_unlock(&zone->lock);
>>  }
>>  
>> @@ -5987,8 +5989,6 @@ __offline_isolated_pages(unsigned long start_pfn, unsigned long end_pfn)
>>  		list_del(&page->lru);
>>  		rmv_page_order(page);
>>  		zone->free_area[order].nr_free--;
>> -		__mod_zone_page_state(zone, NR_FREE_PAGES,
>> -				      - (1UL << order));
>>  		for (i = 0; i < (1 << order); i++)
>>  			SetPageReserved((page+i));
>>  		pfn += (1 << order);
>>
> 
>