linux-kernel - Re: [PATCH] vmscan: make kswapd use a correct order

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Message-ID: <20101203121153.GB13268@csn.ul.ie>
Date:	Fri, 3 Dec 2010 12:11:53 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Shaohua Li <shaohua.li@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] vmscan: make kswapd use a correct order

On Fri, Dec 03, 2010 at 01:00:49AM +0900, Minchan Kim wrote:
> If we wake up prematurely, it means we should keep going on
> reclaiming not new order page but at old order page.
> Sometime new order can be smaller than old order by below
> race so it could make failure of old order page reclaiming.
> 
> T0: Task 1 wakes up kswapd with order-3
> T1: So, kswapd starts to reclaim pages using balance_pgdat
> T2: Task 2 wakes up kswapd with order-2 because pages reclaimed
> 	by T1 are consumed quickly.
> T3: kswapd exits balance_pgdat and will do following:
> T4-1: In beginning of kswapd's loop, pgdat->kswapd_max_order will
> 	be reset with zero.
> T4-2: 'order' will be set to pgdat->kswapd_max_order(0), since it
>         enters the false branch of 'if (order (3) < new_order (2))'
> T4-3: If previous balance_pgdat can't meet requirement of order-2
> 	free pages by high watermark, it will start reclaiming again.
>         So balance_pgdat will use order-0 to do reclaim while it
> 	really should use order-2 at the moment.
> T4-4: At last, Task 1 can't get the any page if it wanted with
> 	GFP_ATOMIC.
> 
> Reported-by: Shaohua Li <shaohua.li@...el.com>
> Signed-off-by: Minchan Kim <minchan.kim@...il.com>
> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Reviewed-by: Shaohua Li <shaohua.li@...el.com>
> Cc: Mel Gorman <mel@....ul.ie>

Acked-by: Mel Gorman <mel@....ul.ie>

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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