linux-kernel - Re: [PATCH 12/25] Remove a branch by assuming __GFP_HIGH == ALLOC

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Message-ID: <20090421084519.GE12713@csn.ul.ie>
Date:	Tue, 21 Apr 2009 09:45:19 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Pekka Enberg <penberg@...helsinki.fi>
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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/25] Remove a branch by assuming __GFP_HIGH ==
	ALLOC_HIGH

On Tue, Apr 21, 2009 at 10:46:22AM +0300, Pekka Enberg wrote:
> On Mon, 2009-04-20 at 23:19 +0100, Mel Gorman wrote:
> > Allocations that specify __GFP_HIGH get the ALLOC_HIGH flag. If these
> > flags are equal to each other, we can eliminate a branch.
> > 
> > [akpm@...ux-foundation.org: Suggested the hack]
> 
> Yikes!
> 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > ---
> >  mm/page_alloc.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 51e1ded..b13fc29 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1639,8 +1639,8 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> >  	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
> >  	 * set both ALLOC_HARDER (!wait) and ALLOC_HIGH (__GFP_HIGH).
> >  	 */
> > -	if (gfp_mask & __GFP_HIGH)
> > -		alloc_flags |= ALLOC_HIGH;
> > +	VM_BUG_ON(__GFP_HIGH != ALLOC_HIGH);
> > +	alloc_flags |= (gfp_mask & __GFP_HIGH);
> 
> Shouldn't you then also change ALLOC_HIGH to use __GFP_HIGH or at least
> add a comment somewhere?
> 

That might break in weird ways if __GFP_HIGH changes in value then. I
can add a comment though

/*
 * __GFP_HIGH is assumed to be the same as ALLOC_HIGH to save a branch.
 * Check for DEBUG_VM that the assumption is still correct. It cannot be
 * checked at compile-time due to casting
 */

?

> >  
> >  	if (!wait) {
> >  		alloc_flags |= ALLOC_HARDER;
> 

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