Re: Suspicious compilation warning

[Russell King - ARM Linux]

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@xxxxxxxxxxxxxxxxxxxxx> 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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