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Date: Thu, 21 Feb 2013 14:44:30 -0800
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCHv6 1/8] zsmalloc: add to mm/
Message-Id: <20130221144430.2d3d77fc.akpm@linux-foundation.org>
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To: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Cody P Schafer <cody@linux.vnet.ibm.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Nitin Gupta <ngupta@vflare.org>, Minchan Kim <minchan@kernel.org>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Dan Magenheimer <dan.magenheimer@oracle.com>, Robert Jennings <rcj@linux.vnet.ibm.com>, Jenifer Hopper <jhopper@us.ibm.com>, Mel Gorman <mgorman@suse.de>, Johannes Weiner <jweiner@redhat.com>, Rik van Riel <riel@redhat.com>, Larry Woodman <lwoodman@redhat.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Dave Hansen <dave@linux.vnet.ibm.com>, Joe Perches <joe@perches.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org

On Thu, 21 Feb 2013 16:21:37 -0600
Seth Jennings <sjenning@linux.vnet.ibm.com> wrote:

> On 02/21/2013 02:36 PM, Cody P Schafer wrote:
> > On Wed, Feb 20, 2013 at 04:04:41PM -0600, Seth Jennings wrote:
> >> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> >> +#define MAX(a, b) ((a) >= (b) ? (a) : (b))
> >> +/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
> >> +#define ZS_MIN_ALLOC_SIZE \
> >> +	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
> > 
> > Could you use the max(a,b) defined in include/linux/kernel.h?
> > 
> 
> Andrew Morton made the same point.  We can't use max() or max_t()
> because the value of ZS_MIN_ALLOC_SIZE is used to derive the value of
> ZS_SIZE_CLASSES which is used to size an array in struct zs_pool.
> 
> So the expression must be completely evaluated to a number by the
> precompiler.

Well yes, but the kernel doesn't need eight(!) separate
implementations of

#define MAX(a, b) ((a) > (b) ? (a) : (b))

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