[RFC]about commit "[PATCH] Align the node_mem_map endpoints to a MAX_ORDER boundary"

[Wang YanQing <udknight@gmail.com>]

Hi Mel Gorman and all, could you explain the code snippet below in
commit e984bb43f7450312ba66fe0e67a99efa6be3b246
"[PATCH] Align the node_mem_map endpoints to a MAX_ORDER boundary"
this commit had getted your ack-by.

"
start = pgdat->node_start_pfn & ~(MAX_ORDER_NR_PAGES - 1);
end = pgdat->node_start_pfn + pgdat->node_spanned_pages;
end = ALIGN(end, MAX_ORDER_NR_PAGES);
size =  (end - start) * sizeof(struct page);
map = alloc_remap(pgdat->node_id, size);
if (!map)
        map = alloc_bootmem_node(pgdat, size);
pgdat->node_mem_map = map + (pgdat->node_start_pfn - start);
"


MY QUESTION IS WHY WE NEED THIS TWO LINES BELOW:

"
start = pgdat->node_start_pfn & ~(MAX_ORDER_NR_PAGES - 1);
"
and
"
pgdat->node_mem_map = map + (pgdat->node_start_pfn - start);
"

Maybe we don't need this trick and can save some hundred bytes.

Thanks.

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