linux-kernel - Re: Suspicious compilation warning

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Date:	Thu, 6 May 2010 10:24:06 +0900
From:	Kyungmin Park <kmpark@...radead.org>
To:	Sergei Shtylyov <sshtylyov@...sta.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Marcelo Jimenez <mroberto@...i.cetuc.puc-rio.br>,
	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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Yinghai Lu <yinghai@...nel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Suspicious compilation warning

On Wed, May 5, 2010 at 7:06 PM, Sergei Shtylyov <sshtylyov@...sta.com> wrote:
> Hello.
>
> Russell King - ARM Linux 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)
>>
>
>  There's DIV_ROUND_UP() macro for this kind of calculation.

Hi,

It tested with my board and working.
Just curious. If NR_SECTION_ROOTS is zero and uninitialized then
what's problem? Since we boot and working without patch.

Thank you,
Kyungmin Park
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