linux-kernel - Re: Suspicious compilation warning

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Message-ID: <20100420230719.GB1432@n2100.arm.linux.org.uk>
Date:	Wed, 21 Apr 2010 00:07:19 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Marcelo Jimenez <mroberto@...i.cetuc.puc-rio.br>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Suspicious compilation warning

On Tue, Apr 20, 2010 at 03:51:22PM -0700, Andrew Morton wrote:
> On Mon, 19 Apr 2010 20:27:43 -0300
> Marcelo Jimenez <mroberto@...i.cetuc.puc-rio.br> wrote:
> 
> > I get this warning while compiling for ARM/SA1100:
> > 
> > mm/sparse.c: In function '__section_nr':
> > mm/sparse.c:135: warning: 'root' is used uninitialized in this function
> > 
> > With a small patch in fs/proc/meminfo.c, I find that NR_SECTION_ROOTS
> > is zero, which certainly explains the warning.
> > 
> > # cat /proc/meminfo
> > NR_SECTION_ROOTS=0
> > NR_MEM_SECTIONS=32
> > SECTIONS_PER_ROOT=512
> > SECTIONS_SHIFT=5
> > MAX_PHYSMEM_BITS=32
> 
> hm, who owns sparsemem nowadays? Nobody identifiable.
> 
> Does it make physical sense to have SECTIONS_PER_ROOT > NR_MEM_SECTIONS?

Well, it'll be about this number on everything using sparsemem extreme:

#define SECTIONS_PER_ROOT       (PAGE_SIZE / sizeof (struct mem_section))

and with only 32 sections, this is going to give a NR_SECTION_ROOTS value
of zero.  I think the calculation of NR_SECTIONS_ROOTS is wrong.

#define NR_SECTION_ROOTS        (NR_MEM_SECTIONS / SECTIONS_PER_ROOT)

Clearly if we have 1 mem section, we want to have one section root, so
I think this division should round up any fractional part, thusly:

#define NR_SECTION_ROOTS        ((NR_MEM_SECTIONS + SECTIONS_PER_ROOT - 1) / SECTIONS_PER_ROOT)
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