[PATCH] MM: Fix NR_SECTION_ROOTS == 0 when using using sparsemem extreme.

[PATCH] MM: Fix NR_SECTION_ROOTS == 0 when using using sparsemem extreme.

[Marcelo Roberto Jimenez]

Got this while compiling for ARM/SA1100:

mm/sparse.c: In function '__section_nr':
mm/sparse.c:135: warning: 'root' is used uninitialized in this function

This patch follows Russell King's suggestion for a new calculation for
NR_SECTION_ROOTS. Thanks also to Sergei Shtylyov for pointing out the
existence of the macro DIV_ROUND_UP.

Signed-off-by: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
---
 include/linux/mmzone.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index cf9e458..e90ad64 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -972,7 +972,7 @@ struct mem_section {
 #endif
 
 #define SECTION_NR_TO_ROOT(sec)	((sec) / SECTIONS_PER_ROOT)
-#define NR_SECTION_ROOTS	(NR_MEM_SECTIONS / SECTIONS_PER_ROOT)
+#define NR_SECTION_ROOTS	DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT)
 #define SECTION_ROOT_MASK	(SECTIONS_PER_ROOT - 1)
 
 #ifdef CONFIG_SPARSEMEM_EXTREME
-- 
1.7.1

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Re: [PATCH] MM: Fix NR_SECTION_ROOTS == 0 when using using sparsemem extreme.

[Marcelo Jimenez]

I have done a limited test of the patch, I don't think the actual code
has been reached, any help would be appreciated.

I did a small patch to /proc/meminfo to check that NR_SECTION_ROOTS is
now non-zero:

$ cat /proc/meminfo
NR_SECTION_ROOTS=1
NR_MEM_SECTIONS=32
SECTIONS_PER_ROOT=512
SECTIONS_SHIFT=5
MAX_PHYSMEM_BITS=32
SECTION_SIZE_BITS=27
...

On Wed, May 5, 2010 at 18:02, Marcelo Roberto Jimenez
<mroberto@cpti.cetuc.puc-rio.br> wrote:
> Got this while compiling for ARM/SA1100:
>
> mm/sparse.c: In function '__section_nr':
> mm/sparse.c:135: warning: 'root' is used uninitialized in this function
>
> This patch follows Russell King's suggestion for a new calculation for
> NR_SECTION_ROOTS. Thanks also to Sergei Shtylyov for pointing out the
> existence of the macro DIV_ROUND_UP.
>
> Signed-off-by: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>

Re: [PATCH] MM: Fix NR_SECTION_ROOTS == 0 when using using sparsemem extreme.

[Atsushi Nemoto]

On Wed,  5 May 2010 18:02:46 -0300, Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br> wrote:
> Got this while compiling for ARM/SA1100:
> 
> mm/sparse.c: In function '__section_nr':
> mm/sparse.c:135: warning: 'root' is used uninitialized in this function
> 
> This patch follows Russell King's suggestion for a new calculation for
> NR_SECTION_ROOTS. Thanks also to Sergei Shtylyov for pointing out the
> existence of the macro DIV_ROUND_UP.

JFYI, This fix is not just silence the warning, fix a real problem.

Without this fix, mem_section[] might have 0 size so mem_section[0]
will share other variable area.  For example, I got:

c030c700 b __warned.16478
c030c700 B mem_section
c030c701 b __warned.16483

This might cause very strange behavior.  Your patch actually fixes it.
Thank you.

---
Atsushi Nemoto

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