linux-kernel - Re: [PATCH 08/10] vmscan: isolated_lru_pages() stop neighbour search if neighbour cannot be isolated

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Message-ID: <20100908131404.GA12660@localhost>
Date:	Wed, 8 Sep 2010 21:14:04 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 08/10] vmscan: isolated_lru_pages() stop neighbour
 search if neighbour cannot be isolated

On Wed, Sep 08, 2010 at 08:50:44PM +0800, Mel Gorman wrote:
> On Wed, Sep 08, 2010 at 07:37:34PM +0800, Wu Fengguang wrote:
> > On Mon, Sep 06, 2010 at 06:47:31PM +0800, Mel Gorman wrote:
> > > From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > > 
> > > isolate_lru_pages() does not just isolate LRU tail pages, but also isolate
> > > neighbour pages of the eviction page. The neighbour search does not stop even
> > > if neighbours cannot be isolated which is excessive as the lumpy reclaim will
> > > no longer result in a successful higher order allocation. This patch stops
> > > the PFN neighbour pages if an isolation fails and moves on to the next block.
> > > 
> > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > > ---
> > >  mm/vmscan.c |   24 ++++++++++++++++--------
> > >  1 files changed, 16 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 64f9ca5..ff52b46 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -1047,14 +1047,18 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> > >  				continue;
> > >  
> > >  			/* Avoid holes within the zone. */
> > > -			if (unlikely(!pfn_valid_within(pfn)))
> > > +			if (unlikely(!pfn_valid_within(pfn))) {
> > > +				nr_lumpy_failed++;
> > >  				break;
> > > +			}
> > >  
> > >  			cursor_page = pfn_to_page(pfn);
> > >  
> > >  			/* Check that we have not crossed a zone boundary. */
> > > -			if (unlikely(page_zone_id(cursor_page) != zone_id))
> > > -				continue;
> > > +			if (unlikely(page_zone_id(cursor_page) != zone_id)) {
> > > +				nr_lumpy_failed++;
> > > +				break;
> > > +			}
> > >  
> > >  			/*
> > >  			 * If we don't have enough swap space, reclaiming of
> > > @@ -1062,8 +1066,10 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> > >  			 * pointless.
> > >  			 */
> > >  			if (nr_swap_pages <= 0 && PageAnon(cursor_page) &&
> > > -					!PageSwapCache(cursor_page))
> > > -				continue;
> > > +			    !PageSwapCache(cursor_page)) {
> > > +				nr_lumpy_failed++;
> > > +				break;
> > > +			}
> > >  
> > >  			if (__isolate_lru_page(cursor_page, mode, file) == 0) {
> > >  				list_move(&cursor_page->lru, dst);
> > > @@ -1074,9 +1080,11 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> > >  					nr_lumpy_dirty++;
> > >  				scan++;
> > >  			} else {
> > > -				if (mode == ISOLATE_BOTH &&
> > > -						page_count(cursor_page))
> > > -					nr_lumpy_failed++;
> > > +				/* the page is freed already. */
> > > +				if (!page_count(cursor_page))
> > > +					continue;
> > > +				nr_lumpy_failed++;
> > > +				break;
> > >  			}
> > >  		}
> > 
> > The many nr_lumpy_failed++ can be moved here:
> > 
> >                 if (pfn < end_pfn)
> >                         nr_lumpy_failed++;
> > 
> 
> Because the break stops the loop iterating, is there an advantage to
> making it a pfn check instead? I might be misunderstanding your
> suggestion.

The complete view in my mind is

                for (; pfn < end_pfn; pfn++) {
                        if (failed 1)
                                break;
                        if (failed 2)
                                break;
                        if (failed 3)
                                break;
                }
                if (pfn < end_pfn)
                        nr_lumpy_failed++;

Sure it just reduces several lines of code :)

Thanks,
Fengguang
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