linux-kernel - Re: [PATCH 21/25] Use allocation flags as an index to the zone watermark

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Message-ID: <alpine.DEB.2.00.0904211739520.31232@chino.kir.corp.google.com>
Date:	Tue, 21 Apr 2009 17:41:53 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	Mel Gorman <mel@....ul.ie>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 21/25] Use allocation flags as an index to the zone
 watermark

On Wed, 22 Apr 2009, KOSAKI Motohiro wrote:

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 376d848..e61867e 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1157,10 +1157,13 @@ failed:
> >  	return NULL;
> >  }
> >  
> > -#define ALLOC_NO_WATERMARKS	0x01 /* don't check watermarks at all */
> > -#define ALLOC_WMARK_MIN		0x02 /* use pages_min watermark */
> > -#define ALLOC_WMARK_LOW		0x04 /* use pages_low watermark */
> > -#define ALLOC_WMARK_HIGH	0x08 /* use pages_high watermark */
> > +/* The WMARK bits are used as an index zone->pages_mark */
> > +#define ALLOC_WMARK_MIN		0x00 /* use pages_min watermark */
> > +#define ALLOC_WMARK_LOW		0x01 /* use pages_low watermark */
> > +#define ALLOC_WMARK_HIGH	0x02 /* use pages_high watermark */
> > +#define ALLOC_NO_WATERMARKS	0x08 /* don't check watermarks at all */
> > +#define ALLOC_WMARK_MASK	0x07 /* Mask to get the watermark bits */
> 
> the mask only use two bit. but mask definition is three bit (0x07), why?
> 

I think it would probably be better to simply use

	#define ALLOC_WMARK_MASK	(ALLOC_NO_WATERMARKS - 1)

here and define ALLOC_NO_WATERMARKS to be 0x04.
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