linux-kernel - Re: [PATCH 1/4] vmscan: change the number of the unmapped files in zone reclaim

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Message-ID: <20090518035319.GA7940@localhost>
Date:	Mon, 18 May 2009 11:53:19 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 1/4] vmscan: change the number of the unmapped files in
	zone reclaim

On Mon, May 18, 2009 at 11:35:31AM +0800, KOSAKI Motohiro wrote:
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -2397,6 +2397,7 @@ static int __zone_reclaim(struct zone *z
> >>               .isolate_pages = isolate_pages_global,
> >>       };
> >>       unsigned long slab_reclaimable;
> >> +     long nr_unmapped_file_pages;
> >>
> >>       disable_swap_token();
> >>       cond_resched();
> >> @@ -2409,9 +2410,11 @@ static int __zone_reclaim(struct zone *z
> >>       reclaim_state.reclaimed_slab = 0;
> >>       p->reclaim_state = &reclaim_state;
> >>
> >> -     if (zone_page_state(zone, NR_FILE_PAGES) -
> >> -             zone_page_state(zone, NR_FILE_MAPPED) >
> >> -             zone->min_unmapped_pages) {
> >> +     nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
> >> +                              zone_page_state(zone, NR_ACTIVE_FILE) -
> >> +                              zone_page_state(zone, NR_FILE_MAPPED);
> >
> > This can possibly go negative.
> 
> Is this a problem?
> negative value mean almost pages are mapped. Thus
> 
>   (nr_unmapped_file_pages > zone->min_unmapped_pages)  => 0
> 
> is ok, I think.

I wonder why you didn't get a gcc warning, because zone->min_unmapped_pages
is a "unsigned long".

Anyway, add a simple note to the code if it works *implicitly*?

Thanks,
Fengguang

> >
> >> +     if (nr_unmapped_file_pages > zone->min_unmapped_pages) {
> >>               /*
> >>                * Free memory by calling shrink zone with increasing
> >>                * priorities until we have enough memory freed.
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