linux-kernel - Re: [PATCH v5 14/14] memory-hotplug: free node

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Date:	Mon, 07 Jan 2013 14:30:37 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wen Congyang <wency@...fujitsu.com>
CC:	Tang Chen <tangchen@...fujitsu.com>, akpm@...ux-foundation.org,
	rientjes@...gle.com, liuj97@...il.com, len.brown@...el.com,
	benh@...nel.crashing.org, paulus@...ba.org, cl@...ux.com,
	minchan.kim@...il.com, kosaki.motohiro@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, wujianguo@...wei.com,
	hpa@...or.com, linfeng@...fujitsu.com, laijs@...fujitsu.com,
	mgorman@...e.de, yinghai@...nel.org, x86@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-acpi@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-ia64@...r.kernel.org, cmetcalf@...era.com,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH v5 14/14] memory-hotplug: free node_data when a node is
 offlined

(2012/12/30 15:02), Wen Congyang wrote:
> At 12/28/2012 08:28 AM, Kamezawa Hiroyuki Wrote:
>> (2012/12/27 21:16), Wen Congyang wrote:
>>> At 12/26/2012 11:55 AM, Kamezawa Hiroyuki Wrote:
>>>> (2012/12/24 21:09), Tang Chen wrote:
>>>>> From: Wen Congyang <wency@...fujitsu.com>
>>>>>
>>>>> We call hotadd_new_pgdat() to allocate memory to store node_data. So we
>>>>> should free it when removing a node.
>>>>>
>>>>> Signed-off-by: Wen Congyang <wency@...fujitsu.com>
>>>>
>>>> I'm sorry but is it safe to remove pgdat ? All zone cache and zonelists are
>>>> properly cleared/rebuilded in synchronous way ? and No threads are visinting
>>>> zone in vmscan.c ?
>>>
>>> We have rebuilt zonelists when a zone has no memory after offlining some pages.
>>>
>>
>> How do you guarantee that the address of pgdat/zone is not on stack of any kernel
>> threads or other kernel objects without reference counting or other syncing method ?
> 
> No way to guarentee this. But, the kernel should not use the address of pgdat/zone when
> it is offlined.
> 
> Hmm, what about this: reuse the memory when the node is onlined again?
> 

That's the only way which we can go now. Please don't free it.

Thanks,
-Kame


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