Re: [2/3] add memory present for ppc64

[Dave Hansen <haveblue@us.ibm.com>]

On Wed, 2005-05-04 at 21:31 -0500, Olof Johansson wrote:
> On Wed, May 04, 2005 at 09:29:57PM +0100, Andy Whitcroft wrote:
> > diff -X /home/apw/brief/lib/vdiff.excl -rupN reference/arch/ppc64/Kconfig current/arch/ppc64/Kconfig
> > --- reference/arch/ppc64/Kconfig	2005-05-04 20:54:50.000000000 +0100
> > +++ current/arch/ppc64/Kconfig	2005-05-04 20:54:50.000000000 +0100
> > @@ -212,8 +212,8 @@ config ARCH_FLATMEM_ENABLE
> >  source "mm/Kconfig"
> >  
> >  config HAVE_ARCH_EARLY_PFN_TO_NID
> > -	bool
> > -	default y
> > +	def_bool y
> > +	depends on NEED_MULTIPLE_NODES
> 
> Ok, time to show my lack of undestanding here, but when can we ever be
> CONFIG_NUMA and NOT need multiple nodes?

NEED_MULTIPLE_NODES is for DISCONTIG || NUMA.  It is a blanket config
option that helps us separate those two very intertwined options.

> > @@ -481,6 +483,7 @@ static void __init setup_nonnuma(void)
> >  
> >  	for (i = 0 ; i < top_of_ram; i += MEMORY_INCREMENT)
> >  		numa_memory_lookup_table[i >> MEMORY_INCREMENT_SHIFT] = 0;
> > +	memory_present(0, 0, init_node_data[0].node_end_pfn);
> 
> Isn't the memory_present stuff and numa_memory_lookup_table two
> implementations doing the same thing (mapping memory to nodes)?

They have similar functions: record the physical layout of the system.
But, memory_present() is for sparsemem, which basically implements
pfn_to_page() and page_to_pfn().

The numa_memory_lookup_table[] is used for pfn_to_nid(), which is
actually orthogonal to what sparsemem needs.

> Can we kill numa_memory_lookup_table with this?

Nope, we still need it for pfn_to_nid().  We could possibly replace the
current implementation like this:

#define pfn_to_nid(pfn)
page_zone(__pfn_to_section(pfn)->section_mem_map[pfn])->zone_pgdat->node_id

But, that might have a few performance implications :)  There are
certainly some options that sparsemem opens up here, and I hope that we
explore them further as we move away from discontig.

We could even do something like store the nid directly in the
mem_section.  But, as I said, that's an optimization that can come
later.

-- Dave

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