linux-kernel - Re: node-hotplug: is memset 0 safe in try_offline

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Date:	Wed, 4 Mar 2015 11:56:32 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
CC:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tang Chen <tangchen@...fujitsu.com>,
	Yinghai Lu <yinghai@...nel.org>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Toshi Kani <toshi.kani@...com>, Mel Gorman <mgorman@...e.de>,
	Tejun Heo <tj@...nel.org>, Xiexiuqi <xiexiuqi@...wei.com>,
	Hanjun Guo <guohanjun@...wei.com>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: node-hotplug: is memset 0 safe in try_offline_node()?

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 (RCU？) 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));
        }
 
        /* we can use NODE_DATA(nid) from here */
@@ -2021,15 +2024,6 @@ void try_offline_node(int nid)
 
        /* notify that the node is down */
        call_node_notify(NODE_DOWN, (void *)(long)nid);
-
-       /*
-        * Since there is no way to guarentee the address of pgdat/zone is not
-        * on stack of any kernel threads or used by other kernel objects
-        * without reference counting or other symchronizing method, do not
-        * reset node_data and free pgdat here. Just reset it to 0 and reuse
-        * the memory when the node is online again.
-        */
-       memset(pgdat, 0, sizeof(*pgdat));
 }
 EXPORT_SYMBOL(try_offline_node);
 

> 
> Thanks,
> Xishi Qiu
> 
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