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

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Date:	Fri, 7 Sep 2012 12:47:16 +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,

2012/09/07 8:37, Minchan Kim wrote:
> Hello Yasuaki,
>
> On Thu, Sep 06, 2012 at 06:14:39PM +0900, Yasuaki Ishimatsu wrote:
>> 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
>
> It seems you are misunderstanding my point.
> Before my patch, Yes. It could be allocated by someone but
> after my patch, it couldn't be allocated and inconsistency between
> pageblock's migratetype and page's migratetype is detected so that
> memory offline would be just failed simply.

Thank you for your explanation.
My concern disappeared.

Thanks,
Yasuaki Ishimatsu

>
>> 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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>


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