Om. wrote:
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 have a hard
time visualizing it. 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.
Is my understanding correct?
(Maybe one can find the page using the node_mem_map as well, but that
is another question.)
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.
I think I could decipher it.
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.
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