Re: Turning on Sparsemem

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Tue, 2008-08-05 at 14:39 -0700, C Michael Sundius wrote:
> Hi Andy and Dave,
> 
> I turned on sparsemem as you described before. I am crashing in
> the mem_init() function when I try a call to pfn_to_page().
> 
> I've noticed that that macro uses the sparsemem macro 
> __pfn_to_section(pfn) and
> that intern calls __nr_to_section(nr). That finally looks at the 
> mem_section[] variable.
> well.. this returns NULL since it seems as though my mem_section[] array 
> looks
> to be not initialized correctly.
> 
> QUESTION: where does this array get initialized. I've looked through the 
> code and
> can't seem to see how that is initialized.

My first guess is that you're missing a call to sparse_init() in your
architecture-specific code.  On x86_32, we do that in paging_init(),
just before zone_sizes_init() (arch/x86/mm/init_32.c).

Before you call this, you'll also have to call memory_present(...) on
the memory that you do have.  But, you should probably already have done
that.

> recall I'm using mips32 processor, but I've looked in all the processors.
> it seems as though sparse_init() and memory present() both use 
> __nr_to_section()
> and thus would require mem_section[] to be set up already.

__nr_to_section() is special.  It takes a section number and just gives
you back the 'struct mem_section'.  It doesn't actually look into that
mem_section and make sure it is valid, it just locates the data
structure.

-- Dave

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