regarding mem_map in NUMA

["Om.Turyx" <om.turyx@gmail.com>]

Hi,
While going through Mel gorman's book, I read that mem_map for NUMA
systems is  treated as a virtual array at PAGE_OFFSET. I could 
understand the the explanation  as follows.

mem_map[pfn] should not be accessed as it is. calculate the actual
node corresponding to pfn and access the page as
pglist_data[A]->node_zonelists[B]->zone_mem_map[C], where,
A : node_id, calculated using pfn,
B : offset in the zonelist calculated using pfn and,
C : offset in the local mem_map calculated using pfn.

Searching google resulted in http://lwn.net/Articles/9188/ Under other 
memory management work, it states mem_map[x] is not recommended in NUMA. 
Instead pfn_to_page() must be used.
from include/asm-i386/mmzone.h,
#define pfn_to_page(pfn)                        \
({\
    unsigned long __pfn = pfn;\
    int __node  = pfn_to_nid(__pfn);\
    &NODE_DATA(__node)->node_mem_map[node_localnr(__pfn,__node)];\
})
Means, pfn is used to traverse nodeid, and node_mem_map.
But even now I have not found where mem_map is initialized to 
PAGE_OFFSET, neither the significance of it.

Now, why should the mem_map be initialized to PAGE_OFFSET? Where is it
done? In page_alloc.c, I found mem_map = NODE_DATA(0)->node_mem_map;
when CONFIG_FLAT_NODE_MEM_MAP  is defined (non numa case).  If my
understanding is correct, this case should hold for NUMA as well.


Regards,
Om.