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Message-ID: <54F6C809.1080709@jp.fujitsu.com>
Date: Wed, 4 Mar 2015 17:53:29 +0900
From: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
MIME-Version: 1.0
Subject: Re: node-hotplug: is memset 0 safe in try_offline_node()?
References: <54F52ACF.4030103@huawei.com> <54F58AE3.50101@cn.fujitsu.com> <54F66C52.4070600@huawei.com> <54F67376.8050001@huawei.com> <54F68270.5000203@cn.fujitsu.com> <54F6BC43.3000509@huawei.com>
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To: Xishi Qiu <qiuxishi@huawei.com>, Gu Zheng <guz.fnst@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>, Andrew Morton <akpm@linux-foundation.org>, Tang Chen <tangchen@cn.fujitsu.com>, Yinghai Lu <yinghai@kernel.org>, Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Toshi Kani <toshi.kani@hp.com>, Mel Gorman <mgorman@suse.de>, Tejun Heo <tj@kernel.org>, Xiexiuqi <xiexiuqi@huawei.com>, Hanjun Guo <guohanjun@huawei.com>, Li Zefan <lizefan@huawei.com>

On 2015/03/04 17:03, Xishi Qiu wrote:
> On 2015/3/4 11:56, Gu Zheng wrote:
>
>> Hi Xishi,
>> On 03/04/2015 10:52 AM, Xishi Qiu wrote:
>>
>>> On 2015/3/4 10:22, Xishi Qiu wrote:
>>>
>>>> On 2015/3/3 18:20, Gu Zheng wrote:
>>>>
>>>>> Hi Xishi,
>>>>> On 03/03/2015 11:30 AM, Xishi Qiu wrote:
>>>>>
>>>>>> When hot-remove a numa node, we will clear pgdat,
>>>>>> but is memset 0 safe in try_offline_node()?
>>>>>
>>>>> It is not safe here. In fact, this is a temporary solution here.
>>>>> As you know, pgdat is accessed lock-less now, so protection
>>>>> mechanism (RCUi 1/4 ?) is needed to make it completely safe here,
>>>>> but it seems a bit over-kill.
>>>>>
>>>
>>> Hi Gu,
>>>
>>> Can we just remove "memset(pgdat, 0, sizeof(*pgdat));" ?
>>> I find this will be fine in the stress test except the warning
>>> when hot-add memory.
>>
>> As you see, it will trigger the warning in free_area_init_node().
>> Could you try the following patch? It will reset the pgdat before reuse it.
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 1778628..0717649 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1092,6 +1092,9 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>>                          return NULL;
>>
>>                  arch_refresh_nodedata(nid, pgdat);
>> +       } else {
>> +               /* Reset the pgdat to reuse */
>> +               memset(pgdat, 0, sizeof(*pgdat));
>>          }
>
> Hi Gu,
>
> If schedule last a long time, next_zone may be still access the pgdat here,
> so it is not safe enough, right?
>

How about just reseting pgdat->nr_zones and pgdat->classzone_idx to be 0 rather than
memset() ?

It seems breaking pointer information in pgdat is not a choice.
Just proper "values" should be reset.

Thanks,
-Kame



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