linux-kernel - Re: [PATCH v2 3/3] memory-hotplug: bug fix race between isolation and allocation

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Date:	Thu, 6 Sep 2012 18:14:39 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Minchan Kim <minchan@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Xishi Qiu <qiuxishi@...wei.com>,
	Wen Congyang <wency@...fujitsu.com>
Subject: Re: [PATCH v2 3/3] memory-hotplug: bug fix race between isolation
 and allocation

Hi, Minchan,

2012/09/06 16:30, Minchan Kim wrote:
> Hello Yasuaki,
>
> On Thu, Sep 06, 2012 at 04:17:54PM +0900, Yasuaki Ishimatsu wrote:
>> Hi Minchan,
>>
>> 2012/09/06 14:16, Minchan Kim wrote:
>>> Like below, memory-hotplug makes race between page-isolation
>>> and page-allocation so it can hit BUG_ON in __offline_isolated_pages.
>>>
>>> 	CPU A					CPU B
>>>
>>> start_isolate_page_range
>>> set_migratetype_isolate
>>> spin_lock_irqsave(zone->lock)
>>>
>>> 				free_hot_cold_page(Page A)
>>> 				/* without zone->lock */
>>> 				migratetype = get_pageblock_migratetype(Page A);
>>> 				/*
>>> 				 * Page could be moved into MIGRATE_MOVABLE
>>> 				 * of per_cpu_pages
>>> 				 */
>>> 				list_add_tail(&page->lru, &pcp->lists[migratetype]);
>>>
>>> set_pageblock_isolate
>>> move_freepages_block
>>> drain_all_pages
>>>
>>> 				/* Page A could be in MIGRATE_MOVABLE of free_list. */
>>>
>>> check_pages_isolated
>>> __test_page_isolated_in_pageblock
>>> /*
>>>    * We can't catch freed page which
>>>    * is free_list[MIGRATE_MOVABLE]
>>>    */
>>> if (PageBuddy(page A))
>>> 	pfn += 1 << page_order(page A);
>>>
>>> 				/* So, Page A could be allocated */
>>>
>>> __offline_isolated_pages
>>> /*
>>>    * BUG_ON hit or offline page
>>>    * which is used by someone
>>>    */
>>> BUG_ON(!PageBuddy(page A));
>>>
>>> This patch checks page's migratetype in freelist in __test_page_isolated_in_pageblock.
>>> So now __test_page_isolated_in_pageblock can check the page caused by above race and
>>> can fail of memory offlining.
>>>
>>> Signed-off-by: Minchan Kim <minchan@...nel.org>
>>> ---
>>>    mm/page_isolation.c |    5 ++++-
>>>    1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>> index 87a7929..7ba7405 100644
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -193,8 +193,11 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn)
>>>    			continue;
>>>    		}
>>>    		page = pfn_to_page(pfn);
>>> -		if (PageBuddy(page))
>>> +		if (PageBuddy(page)) {
>>> +			if (get_freepage_migratetype(page) != MIGRATE_ISOLATE)
>>> +				break;
>>>    			pfn += 1 << page_order(page);
>>> +		}
>>
>>>    		else if (page_count(page) == 0 &&
>>>    			get_freepage_migratetype(page) == MIGRATE_ISOLATE)
>>
>> When do the if statement, the page may be used by someone.
>
> I can't understand your point.
> We already hold zone->lock so that allocator and this function should be atomic
> when the page is in free_list.
> If I miss something, could you elaborate it more?

According to your description, the page might be allocated by someone
at this point. So some value might be set the page->index by the
intended purpose. Thus page->index has the potential to become
MIGRATE_ISOLATE value.

Thanks,
Yasuaki Ishimatsu   

>
>> In this case, page->index may have some number. If the number is same as
>> MIGRATE_ISOLATE, the code goes worng.
>>
>> Thanks,
>> Yasuaki Ishimatsu
>>
>>>    			pfn += 1;
>>>
>>
>>
>> --
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>


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