对于linux内核来说，整个memory系统初始化由函数free_area_init_node()处理，对于多node的则可以
调用函数free_area_init_nodes()。这些函数通过进步调用free_area_init_core()完成对整个node的初始
化。
函数free_area_init_node()主要根据提供的nid和每个zone大小对node下的各个zone初始化，整个node的初始
页帧由参数node_start_pfn给出。
函数calculate_node_totalpages()计算整个node下物理内存多少，确定node下node_spanned_pages和
node_present_pages大小。当然可能情形下确定zone下的spanned_pages和present_pages值。
之后调用函数alloc_node_mem_map()对node下管理的内存分配一些struct page对象进行维护，注意，这里
如果是单node系统，mem_map直接来自node下的node_mem_map。这块物理内存总大小为：
(end - start) * sizeof(struct page)
一般通过memblock来获取。
之后通过函数free_area_init_core()完成进一步初始化。
mm/page_alloc.c
void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
unsigned long node_start_pfn, unsigned long *zholes_size)
{
pg_data_t *pgdat = NODE_DATA(nid);
unsigned long start_pfn = 0;
unsigned long end_pfn = 0;
/* pg_data_t should be reset to zero when it's allocated */
WARN_ON(pgdat->nr_zones || pgdat->kswapd_classzone_idx);
reset_deferred_meminit(pgdat);
pgdat->node_id = nid;
pgdat->node_start_pfn = node_start_pfn;
pgdat->per_cpu_nodestats = NULL;
#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
pr_info("Initmem setup node %d [mem %#018Lx-%#018Lx]\n", nid,
(u64)start_pfn << PAGE_SHIFT,
end_pfn ? ((u64)end_pfn << PAGE_SHIFT) - 1 : 0);
#else
start_pfn = node_start_pfn;
#endif
calculate_node_totalpages(pgdat, start_pfn, end_pfn,zones_size, zholes_size);
alloc_node_mem_map(pgdat);
#ifdef CONFIG_FLAT_NODE_MEM_MAP
printk(KERN_DEBUG "free_area_init_node: node %d, pgdat %08lx, node_mem_map %08lx\n",
nid, (unsigned long)pgdat,
(unsigned long)pgdat->node_mem_map);
#endif
free_area_init_core(pgdat);
}函数free_area_init_core(pgdat)是整个初始化的关键：
/*
* Set up the zone data structures:
* - mark all pages reserved
* - mark all memory queues empty
* - clear the memory bitmaps
*
* NOTE: pgdat should get zeroed by caller.
*/
static void __paginginit free_area_init_core(struct pglist_data *pgdat)
{
enum zone_type j;
int nid = pgdat->node_id;
int ret;
pgdat_resize_init(pgdat);
#ifdef CONFIG_NUMA_BALANCING
spin_lock_init(&pgdat->numabalancing_migrate_lock);
pgdat->numabalancing_migrate_nr_pages = 0;
pgdat->numabalancing_migrate_next_window = jiffies;
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
spin_lock_init(&pgdat->split_queue_lock);
INIT_LIST_HEAD(&pgdat->split_queue);
pgdat->split_queue_len = 0;
#endif
init_waitqueue_head(&pgdat->kswapd_wait);
init_waitqueue_head(&pgdat->pfmemalloc_wait);
#ifdef CONFIG_COMPACTION
init_waitqueue_head(&pgdat->kcompactd_wait);
#endif
pgdat_page_ext_init(pgdat);
spin_lock_init(&pgdat->lru_lock);
lruvec_init(node_lruvec(pgdat));
for (j = 0; j < MAX_NR_ZONES; j++) {
struct zone *zone = pgdat->node_zones + j;
unsigned long size, realsize, freesize, memmap_pages;
unsigned long zone_start_pfn = zone->zone_start_pfn;
size = zone->spanned_pages;
realsize = freesize = zone->present_pages;
/*
* Adjust freesize so that it accounts for how much memory
* is used by this zone for memmap. This affects the watermark
* and per-cpu initialisations
*/
memmap_pages = calc_memmap_size(size, realsize);
if (!is_highmem_idx(j)) {
if (freesize >= memmap_pages) {
freesize -= memmap_pages;
if (memmap_pages)
printk(KERN_DEBUG
" %s zone: %lu pages used for memmap\n",
zone_names[j], memmap_pages);
} else
pr_warn(" %s zone: %lu pages exceeds freesize %lu\n",
zone_names[j], memmap_pages, freesize);
}
/* Account for reserved pages */
if (j == 0 && freesize > dma_reserve) {
freesize -= dma_reserve;
printk(KERN_DEBUG " %s zone: %lu pages reserved\n",
zone_names[0], dma_reserve);
}
if (!is_highmem_idx(j))
nr_kernel_pages += freesize;
/* Charge for highmem memmap if there are enough kernel pages */
else if (nr_kernel_pages > memmap_pages * 2)
nr_kernel_pages -= memmap_pages;
nr_all_pages += freesize;
/*
* Set an approximate value for lowmem here, it will be adjusted
* when the bootmem allocator frees pages into the buddy system.
* And all highmem pages will be managed by the buddy system.
*/
zone->managed_pages = is_highmem_idx(j) ? realsize : freesize;
#ifdef CONFIG_NUMA
zone->node = nid;
#endif
zone->name = zone_names[j];
zone->zone_pgdat = pgdat;
spin_lock_init(&zone->lock);
zone_seqlock_init(zone);
zone_pcp_init(zone);
if (!size)
continue;
set_pageblock_order();
setup_usemap(pgdat, zone, zone_start_pfn, size);
ret = init_currently_empty_zone(zone, zone_start_pfn, size);
BUG_ON(ret);
memmap_init(size, nid, j, zone_start_pfn);
}
