[Crash-utility] ARM support for CONFIG_SPARSEMEM: (was Re: DDimage)

[Dave Anderson]

----- Original Message -----
> Hi Dave,
> >but the following caliculation of NR_SECTION_ROOTS() looks suspicious
> >to me.
> >#define NR_SECTION_ROOTS() (NR_MEM_SECTIONS() / SECTIONS_PER_ROOT())
> >
> >Something like (((NR_MEM_SECTIONS() - 1)/ SECTIONS_PER_ROOT()) + 1) ?
> 
> Now I am quite sure the definition of NR_SECTION_ROOTS() is wrong.
> In the upstream kernel, I found the following 
> include/linux/mmzone.h
> ...
> 994 #define SECTION_NR_TO_ROOT(sec) ((sec) / SECTIONS_PER_ROOT)
> 995 #define NR_SECTION_ROOTS DIV_ROUND_UP(NR_MEM_SECTIONS, SECTIONS_PER_ROOT)
> 996 #define SECTION_ROOT_MASK (SECTIONS_PER_ROOT - 1)
> 
> and include/kernel.h DIV_ROUND_UP is defined as follows,
> 58 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> 
> I hope you can take care of this.
> 
> By the way, I think _SECTION_SIZE_BITS issue is a separate issue.
> 
> Best Regard,
> 
> Takuo

Hello Takuo,

Very interesting -- the change to the NR_SECTION_ROOTS() calculation was
only commited upstream *yesterday*:

  commit 0faa56389c793cda7f967117415717bbab24fe4e
  Author: Marcelo Roberto Jimenez <mroberto at cpti.cetuc.puc-rio.br>
  Date:   Mon May 24 14:32:47 2010 -0700
  
      mm: fix NR_SECTION_ROOTS == 0 when using using sparsemem extreme.
      
      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.
      
      Atsushi Nemoto observed:
      : This fix doesn't just silence the warning - it fixes 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.
      
      Signed-off-by: Marcelo Roberto Jimenez <mroberto at cpti.cetuc.puc-rio.br>
      Cc: Atsushi Nemoto <anemo at mba.ocn.ne.jp>
      Cc: KOSAKI Motohiro <kosaki.motohiro at jp.fujitsu.com>
      Cc: Christoph Lameter <cl at linux-foundation.org>
      Cc: Mel Gorman <mel at csn.ul.ie>
      Cc: Minchan Kim <minchan.kim at gmail.com>
      Cc: Yinghai Lu <yinghai at kernel.org>
      Cc: Sergei Shtylyov <sshtylyov at mvista.com>
      Cc: Russell King <rmk at arm.linux.org.uk>
      Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
      Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
  
  diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
  index fd55f72..f6f2c50 100644
  --- a/include/linux/mmzone.h
  +++ b/include/linux/mmzone.h
  @@ -981,7 +981,7 @@ 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
  
I'll test this out, and presuming that nothing breaks w/respect to
backwards compatibility, I'll change the crash version of NR_SECTION_ROOTS.  

But, as you state, the _SECTION_SIZE_BITS is based upon whatever your
kernel is doing.

Thanks,
  Dave