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Subject: Re: [PATCH 12/25] Remove a branch by assuming __GFP_HIGH ==
 ALLOC_HIGH
From: Pekka Enberg <penberg@cs.helsinki.fi>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Lin Ming <ming.m.lin@intel.com>,
       Zhang Yanmin <yanmin_zhang@linux.intel.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <1240266011-11140-13-git-send-email-mel@csn.ul.ie>
References: <1240266011-11140-1-git-send-email-mel@csn.ul.ie>
	 <1240266011-11140-13-git-send-email-mel@csn.ul.ie>
Date: Tue, 21 Apr 2009 10:46:22 +0300
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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@linux-foundation.org: Suggested the hack]

Yikes!

> Signed-off-by: Mel Gorman <mel@csn.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?

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