pg_data_t數據結構2.6.37
這個數據結構用於非一致內存訪問(NUMA)機器，表示比zone更高層次的內存區。
關於NUMA的文章
http://www.ibm.com/developerworks/cn/linux/l-numa/
http://blog.chinaunix.net/uid-7295895-id-3076420.html
NUMA有多個節點，而每個節點內，訪問內存的時間是相同的，不同的節點，訪問內存的時間可以不同。
在NUMA機器上，每個NUMA 節點(node)有一個pg_data_t來描述它的內存佈局。
對於UMA上，只有一個節點的情況，只是contig_page_data的靜態pg_data_t結構。
#define NODE_DATA(nid) (&contig_page_data)
每個節點(node)，有可以分成區（zone)，如ZONE_DMA、ZONE_NORMAL、ZONE_HIGHMEM
每個區有自己的特殊用途，如ZONE_DMA是低端的物理內存，如ISA設備需要用到它；
ZONE_NORMAL的地址可以直接映射到線性地址空間中。
不過對於ARM來說，ZONE_DMA和ZONE_NORMAL應該沒有啥區別。
/*
* The pg_data_t structure is used in machines with CONFIG_DISCONTIGMEM
* (mostly NUMA machines?) to denote a higher-level memory zone than the
* zone denotes.
*
* On NUMA machines, each NUMA node would have a pg_data_t to describe
* it's memory layout.
*
* Memory statistics and page replacement data structures are maintained on a
* per-zone basis.
*/
struct bootmem_data;
typedef struct pglist_data {
struct zone node_zones[MAX_NR_ZONES];
struct zonelist node_zonelists[MAX_ZONELISTS];
int nr_zones; /* 表示該節點的區數 */
#ifdef CONFIG_FLAT_NODE_MEM_MAP /* means !SPARSEMEM */
struct page *node_mem_map;
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
struct page_cgroup *node_page_cgroup;
#endif
#endif
#ifndef CONFIG_NO_BOOTMEM
struct bootmem_data *bdata;/* 指向內存引導程序 */
#endif
#ifdef CONFIG_MEMORY_HOTPLUG
/*
* Must be held any time you expect node_start_pfn, node_present_pages
* or node_spanned_pages stay constant. Holding this will also
* guarantee that any pfn_valid() stays that way.
*
* Nests above zone->lock and zone->size_seqlock.
*/
spinlock_t node_size_lock;
#endif
unsigned long node_start_pfn; /* 節點(node)的開始PFN */
unsigned long node_present_pages; /* total number of physical pages
此節點總的物理頁數
*/
unsigned long node_spanned_pages; /* total size of physical page
range, including holes
此節點物理頁數，包括洞
*/
int node_id; /* node id(NID) 節點的ID號 */
wait_queue_head_t kswapd_wait;/* kswpdN的等待隊列鏈表 */
struct task_struct *kswapd; /* 該節點的kswapdN進程 */
int kswapd_max_order; /* ...*/
} pg_data_t;
node_zones:
該節點的區。
zone_type 區類型
enum zone_type {
#ifdef CONFIG_ZONE_DMA
/*
* ZONE_DMA is used when there are devices that are not able
* to do DMA to all of addressable memory (ZONE_NORMAL). Then we
* carve out the portion of memory that is needed for these devices.
* The range is arch specific.這個範圍是體系結構決定的
*
* Some examples
*
* Architecture Limit
* ---------------------------
* parisc, ia64, sparc <4G
* s390 <2G
* arm Various
* alpha Unlimited or 0-16MB.
*
* i386, x86_64 and multiple other arches
* <16M.
*/
ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
/*
* x86_64 needs two ZONE_DMAs because it supports devices that are
* only able to do DMA to the lower 16M but also 32 bit devices that
* can only do DMA areas below 4G.
*/
ZONE_DMA32,
#endif
/*
* Normal addressable memory is in ZONE_NORMAL. DMA operations can be
* performed on pages in ZONE_NORMAL if the DMA devices support
* transfers to all addressable memory.
* 如果DMA設備支持所有可尋址內存，則DMA操作能在ZONE_NORMAL的頁中執行
*/
ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
/*
* A memory area that is only addressable by the kernel through
* mapping portions into its own address space. This is for example
* used by i386 to allow the kernel to address the memory beyond
* 900MB. The kernel will set up special mappings (page
* table entries on i386) for each page that the kernel needs to
* access.
*/
ZONE_HIGHMEM,
#endif
ZONE_MOVABLE,
__MAX_NR_ZONES
};
node_zonelists:內存分配時，區的順序
start_kernel()==>build_all_zonelists(NULL);===》》build_zonelists
node_mem_map: 指向struct page數組的第一個頁面，即mem_map數組，指向該node所有的頁；
free_area_init_node=>alloc_node_mem_map
mem_map = NODE_DATA(0)->node_mem_map;