linux-kernel - Re: [PATCH 18/22] Use allocation flags as an index to the zone watermark

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Message-ID: <20090423002934.GC26643@csn.ul.ie>
Date:	Thu, 23 Apr 2009 01:29:35 +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>,
	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 18/22] Use allocation flags as an index to the zone
	watermark

On Wed, Apr 22, 2009 at 01:06:07PM -0700, David Rientjes wrote:
> On Wed, 22 Apr 2009, Mel Gorman wrote:
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index b174f2c..6030f49 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1154,10 +1154,15 @@ 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	0x04 /* don't check watermarks at all */
> > +
> > +/* Mask to get the watermark bits */
> > +#define ALLOC_WMARK_MASK	(ALLOC_NO_WATERMARKS-1)
> > +
> >  #define ALLOC_HARDER		0x10 /* try to alloc harder */
> >  #define ALLOC_HIGH		0x20 /* __GFP_HIGH set */
> >  #define ALLOC_CPUSET		0x40 /* check for correct cpuset */
> 
> The watermark flags should probably be members of an anonymous enum since 
> they're being used as an index into an array.  If another watermark were 
> ever to be added it would require a value of 0x03, for instance.
> 
> 	enum {
> 		ALLOC_WMARK_MIN,
> 		ALLOC_WMARK_LOW,
> 		ALLOC_WMARK_HIGH,
> 
> 		ALLOC_WMARK_MASK = 0xf	/* no more than 16 possible watermarks */
> 	};
> 
> This eliminates ALLOC_NO_WATERMARKS and the caller that uses it would 
> simply pass 0.
> 

I'm missing something here. If ALLOC_NO_WATERMARKS was defined as zero
then thing like this break.

        if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
                if (!in_interrupt() &&
                    ((p->flags & PF_MEMALLOC) ||
                     unlikely(test_thread_flag(TIF_MEMDIE))))
                        alloc_flags |= ALLOC_NO_WATERMARKS;
        }

Also, the ALLOC_HARDER and other alloc flags need to be redefined for
ALLOC_WMARK_MASK == 0xf. I know what you are getting at but it's a bit more
involved than you're making out and I'm not seeing an advantage.

> > @@ -1445,12 +1450,7 @@ zonelist_scan:
> >  
> >  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> 
> This would become
> 
> 	if (alloc_flags & ALLOC_WMARK_MASK)
> 

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