linux-kernel - RE: [RFC][PATCH] avoid swapping out with swappiness==0

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Date:	Fri, 2 Mar 2012 18:43:01 -0500
From:	Satoru Moriya <satoru.moriya@....com>
To:	Rik van Riel <riel@...hat.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lwoodman@...hat.com" <lwoodman@...hat.com>,
	"jweiner@...hat.com" <jweiner@...hat.com>,
	"shaohua.li@...el.com" <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: RE: [RFC][PATCH] avoid swapping out with swappiness==0

Hi Rik,

Thank you for reviewing.

On 03/02/2012 05:47 PM, Rik van Riel wrote:
> On 03/02/2012 12:36 PM, Satoru Moriya wrote:
>> @@ -1999,7 +1999,7 @@ out:
>>           unsigned long scan;
>>
>>           scan = zone_nr_lru_pages(mz, lru);
>> -        if (priority || noswap) {
>> +        if (priority || noswap || !vmscan_swappiness(mz, sc)) {
>>               scan>>= priority;
>>               if (!scan&&  force_scan)
>>                   scan = SWAP_CLUSTER_MAX;
> 
> However, I do not understand why we fail to scale the number of pages 
> we want to scan with priority if "noswap".
> 
> For that matter, surely if we do not want to swap out anonymous pages, 
> we WANT to go into this if branch, in order to make sure we set "scan" 
> to 0?
> 
> scan = div64_u64(scan * fraction[file], denominator);
> 
> With your patch and swappiness=0, or no swap space, it looks like we 
> do not zero out "scan" and may end up scanning anonymous pages.

With my patch, if swappiness==0 or noswap==1, fraction[file] is
set to 0. As a result, scan will be set to 0, too.

> Am I overlooking something?  Is this correct?
> 
> I mean, it is Friday and my brain is very full...

Have a nice weekend ;)

Regards,
Satoru