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

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Date:	Tue, 3 Mar 2015 18:20:19 +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>
Subject: Re: node-hotplug: is memset 0 safe in try_offline_node()?

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.

> 
> process A:			offline node XX:
> for_each_populated_zone()
> find online node XX
> cond_resched()
> 				offline cpu and memory, then try_offline_node()
> 				node_set_offline(nid), and memset(pgdat, 0, sizeof(*pgdat))
> access node XX's pgdat
> NULL pointer access error

It's possible, but I did not meet this condition, did you?

Regards,
Gu

> 
> Thanks,
> Xishi Qiu
> 
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