linux-kernel - Re: [PATCH] trivial, memory hotplug: add kswapd_is

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Message-ID: <4FDE8081.1070500@kernel.org>
Date:	Mon, 18 Jun 2012 10:12:33 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
CC:	Jiang Liu <jiang.liu@...wei.com>,
	Keping Chen <chenkeping@...wei.com>,
	Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Jiang Liu <liuj97@...il.com>
Subject: Re: [PATCH] trivial, memory hotplug: add kswapd_is_running() for
 better readability

On 06/17/2012 11:19 AM, David Rientjes wrote:

> On Thu, 14 Jun 2012, Jiang Liu wrote:
> 
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index c84ec68..36249d5 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -301,6 +301,11 @@ static inline void scan_unevictable_unregister_node(struct node *node)
>>  
>>  extern int kswapd_run(int nid);
>>  extern void kswapd_stop(int nid);
>> +static inline bool kswapd_is_running(int nid)
>> +{
>> +	return !!(NODE_DATA(nid)->kswapd);
>> +}
>> +
>>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>>  extern int mem_cgroup_swappiness(struct mem_cgroup *mem);
>>  #else
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 0d7e3ec..88e479d 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -522,7 +522,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages)
>>  	init_per_zone_wmark_min();
>>  
>>  	if (onlined_pages) {
>> -		kswapd_run(zone_to_nid(zone));
>> +		if (!kswapd_is_running(zone_to_nid(zone)))
>> +			kswapd_run(zone_to_nid(zone));
>>  		node_set_state(zone_to_nid(zone), N_HIGH_MEMORY);
>>  	}
>>  
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7585101..3dafdbe 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2941,8 +2941,7 @@ int kswapd_run(int nid)
>>  	pg_data_t *pgdat = NODE_DATA(nid);
>>  	int ret = 0;
>>  
>> -	if (pgdat->kswapd)
>> -		return 0;
>> +	BUG_ON(pgdat->kswapd);
>>  
>>  	pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
>>  	if (IS_ERR(pgdat->kswapd)) {
> 
> This isn't better, there's no functional change and you've just added a 
> second conditional for no reason and an unnecessary kswapd_is_running() 
> function.


Tend to agree.
Now that I think about it, it's enough to add comment.

> 
> More concerning is that online_pages() doesn't check the return value of 
> kswapd_run().  We should probably fail the memory hotplug operation that 
> onlines a new node and doesn't have a kswapd running and cleanup after 
> ourselves in online_pages() with some sane error handling.


Yeb. It's more valuable.

> 
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-- 
Kind regards,
Minchan Kim
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