pg_data_t数据结构2.6.37
这个数据结构用于非一致内存访问(NUMA)机器，表示比zone更高层次的内存区。
关于NUMA的文章
http://www.ibm.com/developerworks/cn/linux/l-numa/
http://blog.chinaunix.net/uid-7295895-id-3076420.html
NUMA有多个节点，而每个节点内，访问内存的时间是相同的，不同的节点，访问内存的时间可以不同。
在NUMA机器上，每个NUMA 节点(node)有一个pg_data_t来描述它的内存布局。
对于UMA上，只有一个节点的情况，只是contig_page_data的静态pg_data_t结构。
#define NODE_DATA(nid) (&contig_page_data)
每个节点(node)，有可以分成区（zone)，如ZONE_DMA、ZONE_NORMAL、ZONE_HIGHMEM
每个区有自己的特殊用途，如ZONE_DMA是低端的物理内存，如ISA设备需要用到它；
ZONE_NORMAL的地址可以直接映射到线性地址空间中。
不过对于ARM来说，ZONE_DMA和ZONE_NORMAL应该没有啥区别。
/*
* 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; /* 表示该节点的区数 */
#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;/* 指向内存引导程序 */
#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; /* 节点(node)的开始PFN */
unsigned long node_present_pages; /* total number of physical pages
此节点总的物理页数
*/
unsigned long node_spanned_pages; /* total size of physical page
range, including holes
此节点物理页数，包括洞
*/
int node_id; /* node id(NID) 节点的ID号 */
wait_queue_head_t kswapd_wait;/* kswpdN的等待队列链表 */
struct task_struct *kswapd; /* 该节点的kswapdN进程 */
int kswapd_max_order; /* ...*/
} pg_data_t;
node_zones:
该节点的区。
zone_type 区类型
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.这个范围是体系结构决定的
*
* 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设备支持所有可寻址内存，则DMA操作能在ZONE_NORMAL的页中执行
*/
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:内存分配时，区的顺序
start_kernel()==>build_all_zonelists(NULL);===》》build_zonelists
node_mem_map: 指向struct page数组的第一个页面，即mem_map数组，指向该node所有的页；
free_area_init_node=>alloc_node_mem_map
mem_map = NODE_DATA(0)->node_mem_map;
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