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Date: Wed, 05 May 2010 14:06:45 +0400
From: Sergei Shtylyov <sshtylyov@mvista.com>
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To: Russell King - ARM Linux <linux@arm.linux.org.uk>
CC: Andrew Morton <akpm@linux-foundation.org>,
       Stephen Rothwell <sfr@canb.auug.org.au>,
       Marcelo Jimenez <mroberto@cpti.cetuc.puc-rio.br>,
       Christoph Lameter <cl@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       Minchan Kim <minchan.kim@gmail.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       "H. Peter Anvin" <hpa@zytor.com>, Yinghai Lu <yinghai@kernel.org>,
       linux-arm-kernel@lists.infradead.org
Subject: Re: Suspicious compilation warning
References: <k2ncecb6d8f1004191627w3cd36450xf797f746460abb09@mail.gmail.com>	<20100420155122.6f2c26eb.akpm@linux-foundation.org> <20100420230719.GB1432@n2100.arm.linux.org.uk>
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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.

WBR, Sergei