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

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Date:	Thu, 5 Mar 2015 17:39:09 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Gu Zheng <guz.fnst@...fujitsu.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>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: node-hotplug: is memset 0 safe in try_offline_node()?

On 2015/3/5 16:26, Gu Zheng wrote:

> Hi Xishi,
> Could you please try the following one?
> It postpones the reset of obsolete pgdat from try_offline_node() to
> hotadd_new_pgdat(), and just resetting pgdat->nr_zones and
> pgdat->classzone_idx to be 0 rather than the whole reset by memset()
> as Kame suggested.
> 
> Regards,
> Gu
> 
> ---
>  mm/memory_hotplug.c |   13 ++++---------
>  1 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1778628..c17eebf 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1092,6 +1092,10 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
>  			return NULL;
>  
>  		arch_refresh_nodedata(nid, pgdat);
> +	} else {
> +		/* Reset the nr_zones and classzone_idx to 0 before reuse */
> +		pgdat->nr_zones = 0;
> +		pgdat->classzone_idx = 0;

Hi Gu,

This is just to avoid the warning, I think it's no meaning.
Here is the changlog from the original patch:

commit 88fdf75d1bb51d85ba00c466391770056d44bc03
    ...
    Warn if memory-hotplug/boot code doesn't initialize pg_data_t with zero
    when it is allocated.  Arch code and memory hotplug already initiailize
    pg_data_t.  So this warning should never happen.  I select fields *randomly*
    near the beginning, middle and end of pg_data_t for checking.
    ...

Thanks,
Xishi Qiu

>  	}
>  
>  	/* we can use NODE_DATA(nid) from here */
> @@ -2021,15 +2025,6 @@ 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);
>  



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