linux-kernel - Re: [RFC] Replace the watermark-related union in struct zone with a watermark[] array

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Date:	Mon, 27 Apr 2009 21:54:00 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Replace the watermark-related union in struct zone with
	a watermark[] array

On Mon, Apr 27, 2009 at 01:48:47PM -0700, David Rientjes wrote:
> On Mon, 27 Apr 2009, Mel Gorman wrote:
> 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index c1fa208..1ff59fd 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -163,6 +163,13 @@ static inline int is_unevictable_lru(enum lru_list l)
> >  #endif
> >  }
> >  
> > +enum zone_watermarks {
> > +	WMARK_MIN,
> > +	WMARK_LOW,
> > +	WMARK_HIGH,
> > +	NR_WMARK
> > +};
> > +
> >  struct per_cpu_pages {
> >  	int count;		/* number of pages in the list */
> >  	int high;		/* high watermark, emptying needed */
> > @@ -275,12 +282,9 @@ struct zone_reclaim_stat {
> >  
> >  struct zone {
> >  	/* Fields commonly accessed by the page allocator */
> > -	union {
> > -		struct {
> > -			unsigned long	pages_min, pages_low, pages_high;
> > -		};
> > -		unsigned long pages_mark[3];
> > -	};
> > +
> > +	/* zone watermarks, indexed with WMARK_LOW, WMARK_MIN and WMARK_HIGH */
> > +	unsigned long watermark[NR_WMARK];
> >  
> >  	/*
> >  	 * We don't know if the memory that we're going to allocate will be freeable
> 
> I thought the suggestion was for something like
> 
> 	#define zone_wmark_min(z)	(z->pages_mark[WMARK_MIN])
> 	...

Was it the only suggestion? I thought just replacing the union with an
array would be an option as well.

The #define approach also requires setter versions like

static inline set_zone_wmark_min(struct zone *z, unsigned long val)
{
	z->pages_mark[WMARK_MIN] = val;
}

and you need one of those for each watermark if you are to avoid weirdness like

zone_wmark_min(z) = val;

which looks all wrong. I felt this approach would look neater and be closer
in appearance to the code that is already there and therefore less surprising.

Would people prefer a getter/setter version?

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