linux-kernel - Re: [PATCH 1/3] mm: page allocator: Update free page counters after pages are placed on the free list

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Message-ID: <20100905180619.GE8384@csn.ul.ie>
Date:	Sun, 5 Sep 2010 19:06:20 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 1/3] mm: page allocator: Update free page counters
	after pages are placed on the free list

On Fri, Sep 03, 2010 at 03:38:59PM -0700, Andrew Morton wrote:
> On Fri,  3 Sep 2010 10:08:44 +0100
> Mel Gorman <mel@....ul.ie> wrote:
> 
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -588,12 +588,12 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  {
> >  	int migratetype = 0;
> >  	int batch_free = 0;
> > +	int freed = count;
> >  
> >  	spin_lock(&zone->lock);
> >  	zone->all_unreclaimable = 0;
> >  	zone->pages_scanned = 0;
> >  
> > -	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
> >  	while (count) {
> >  		struct page *page;
> >  		struct list_head *list;
> > @@ -621,6 +621,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
> >  			trace_mm_page_pcpu_drain(page, 0, page_private(page));
> >  		} while (--count && --batch_free && !list_empty(list));
> >  	}
> > +	__mod_zone_page_state(zone, NR_FREE_PAGES, freed);
> >  	spin_unlock(&zone->lock);
> >  }
> >  
> 
> nit: this is why it's evil to modify the value of incoming args.  It's
> nicer to leave them alone and treat them as const across the lifetime
> of the callee.
> 

Ok, I can see the logic of that.

> Can I do this?
> 
> --- a/mm/page_alloc.c~mm-page-allocator-update-free-page-counters-after-pages-are-placed-on-the-free-list-fix
> +++ a/mm/page_alloc.c
> @@ -588,13 +588,13 @@ static void free_pcppages_bulk(struct zo
>  {
>  	int migratetype = 0;
>  	int batch_free = 0;
> -	int freed = count;
> +	int to_free = count;
>  
>  	spin_lock(&zone->lock);
>  	zone->all_unreclaimable = 0;
>  	zone->pages_scanned = 0;
>  
> -	while (count) {
> +	while (to_free) {
>  		struct page *page;
>  		struct list_head *list;
>  
> @@ -619,9 +619,9 @@ static void free_pcppages_bulk(struct zo
>  			/* MIGRATE_MOVABLE list may include MIGRATE_RESERVEs */
>  			__free_one_page(page, zone, 0, page_private(page));
>  			trace_mm_page_pcpu_drain(page, 0, page_private(page));
> -		} while (--count && --batch_free && !list_empty(list));
> +		} while (--to_free && --batch_free && !list_empty(list));
>  	}
> -	__mod_zone_page_state(zone, NR_FREE_PAGES, freed);
> +	__mod_zone_page_state(zone, NR_FREE_PAGES, count);
>  	spin_unlock(&zone->lock);
>  }

Yes you can. I see no problem with this alteration.

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