linux-kernel - Re: [patch 3/3][rfc] vmscan: batched swap slot allocation

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Date:	Tue, 21 Apr 2009 11:38:30 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 3/3][rfc] vmscan: batched swap slot allocation

On Tue, Apr 21, 2009 at 06:27:08PM +0900, KOSAKI Motohiro wrote:
> > > > -		cond_resched();
> > > > +		if (list_empty(&swap_pages))
> > > > +			cond_resched();
> > > >  
> > > Why this ?
> > 
> > It shouldn't schedule anymore when it's allocated the first swap slot.
> > Another reclaimer could e.g. sleep on the cond_resched() before the
> > loop and when we schedule while having swap slots allocated, we might
> > continue further allocations multiple slots ahead.
> 
> Oops, It seems regression. this cond_resched() intent to
> 
> cond_resched();
> pageout();
> cond_resched();
> pageout();
> cond_resched();
> pageout();

It still does that.  While it collects swap pages (swap_pages list is
non-empty), it doesn't page out.  And if it restarts for unmap and
page-out, the swap_pages list is empty and cond_resched() is called.
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