1、系统中的内存域，使用枚举常量zone_type来表示：
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.
*
* 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.
*/
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
};
（1）ZONE_DMA：当一个外设需要用DMA技术，但又不能映射到所有内存区域【ZONE_NORMAL】时，需要用到ZONE_DMA。在I32/x86_64下，一般不会超过16M；
（2）ZONE_DMA32：在64位机器上，若要支持只能访问4G一下内存的32bit外设，才需要ZONE_DMA32
（3）ZONE_HIGHMEM：对内核来说，如果物理内存比VM的空间还大时，需要使用高端内存【high memory】；高端内存区域的访问，需要内核借助映射机制，将当前需要用的一部分物理内存映射到VM空间。
所以：需要一些数据结构进行内存域管理。
2、每个内存域都关联了一个数组，用来组织属于该内存域的物理内存页【页帧】，对于每个页帧，都分配一个struct page实例以及所需的管理数据。所以，需要一些数据结构进行页帧管理，物理页帧对应的数据结构是：struct page。页帧代表系统内存的最小单位。
3、Linux内存管理充分考虑了UMA和NUMA系统的划分，主要基于以下思想：在UMA系统上，则只用一个NUMA结点来管理整个系统的内存，而内存管理的其他部分则相信它们在管理一个伪NUMA系统。所以，需要一些数据结构进行结点管理。
负责进行结点管理的数据结构为：pg_data_t，定义为：
/* * 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; #ifdef CONFIG_FLAT_NODE_MEM_MAP struct page *node_mem_map; #endif struct bootmem_data *bdata; #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; unsigned long node_present_pages; /* total number of physical pages */ //node中所有存在的可用的物理页的数量 unsigned long node_spanned_pages; /* total size of physical page range, including holes */ //node中所有存在的物理页的总数量，包括空洞 int node_id; wait_queue_head_t kswapd_wait; struct task_struct *kswapd; int kswapd_max_order; } pg_data_t;如果结点的个数多余一个，那么内核会维护各个结点的状态信息，相应的数据结构定义如下（node_set_state和node_clear_state用来处理该位图操作）：
/* * Bitmasks that are kept for all the nodes. */ enum node_states { N_POSSIBLE, /* The node could become online at some point */ N_ONLINE, /* The node is online */ N_NORMAL_MEMORY, /* The node has regular memory */ #ifdef CONFIG_HIGHMEM N_HIGH_MEMORY, /* The node has regular or high memory */ #else N_HIGH_MEMORY = N_NORMAL_MEMORY, #endif N_CPU, /* The node has one or more cpus */ NR_NODE_STATES };4、内存域管理
struct zone { /* Fields commonly accessed by the page allocator */ unsigned long pages_min, pages_low, pages_high; /* * We don't know if the memory that we're going to allocate will be freeable * or/and it will be released eventually, so to avoid totally wasting several * GB of ram we must reserve some of the lower zone memory (otherwise we risk * to run OOM on the lower zones despite there's tons of freeable ram * on the higher zones). This array is recalculated at runtime if the * sysctl_lowmem_reserve_ratio sysctl changes. */ unsigned long lowmem_reserve[MAX_NR_ZONES]; #ifdef CONFIG_NUMA int node; /* * zone reclaim becomes active if more unmapped pages exist. */ unsigned long min_unmapped_pages; unsigned long min_slab_pages; struct per_cpu_pageset *pageset[NR_CPUS]; #else struct per_cpu_pageset pageset[NR_CPUS]; #endif /* * free areas of different sizes */ spinlock_t lock; #ifdef CONFIG_MEMORY_HOTPLUG /* see spanned/present_pages for more description */ seqlock_t span_seqlock; #endif struct free_area free_area[MAX_ORDER]; #ifndef CONFIG_SPARSEMEM /* * Flags for a pageblock_nr_pages block. See pageblock-flags.h. * In SPARSEMEM, this map is stored in struct mem_section */ unsigned long *pageblock_flags; #endif /* CONFIG_SPARSEMEM */ ZONE_PADDING(_pad1_) /* Fields commonly accessed by the page reclaim scanner */ spinlock_t lru_lock; struct list_head active_list; struct list_head inactive_list; unsigned long nr_scan_active; unsigned long nr_scan_inactive; unsigned long pages_scanned; /* since last reclaim */ unsigned long flags; /* zone flags, see below */ /* Zone statistics */ atomic_long_t vm_stat[NR_VM_ZONE_STAT_ITEMS]; /* * prev_priority holds the scanning priority for this zone. It is * defined as the scanning priority at which we achieved our reclaim * target at the previous try_to_free_pages() or balance_pgdat() * invokation. * * We use prev_priority as a measure of how much stress page reclaim is * under - it drives the swappiness decision: whether to unmap mapped * pages. * * Access to both this field is quite racy even on uniprocessor. But * it is expected to average out OK. */ int prev_priority; ZONE_PADDING(_pad2_) /* Rarely used or read-mostly fields */ /* * wait_table -- the array holding the hash table * wait_table_hash_nr_entries -- the size of the hash table array * wait_table_bits -- wait_table_size == (1 << wait_table_bits) * * The purpose of all these is to keep track of the people * waiting for a page to become available and make them * runnable again when possible. The trouble is that this * consumes a lot of space, especially when so few things * wait on pages at a given time. So instead of using * per-page waitqueues, we use a waitqueue hash table. * * The bucket discipline is to sleep on the same queue when * colliding and wake all in that wait queue when removing. * When something wakes, it must check to be sure its page is * truly available, a la thundering herd. The cost of a * collision is great, but given the expected load of the * table, they should be so rare as to be outweighed by the * benefits from the saved space. * * __wait_on_page_locked() and unlock_page() in mm/filemap.c, are the * primary users of these fields, and in mm/page_alloc.c * free_area_init_core() performs the initialization of them. */ wait_queue_head_t * wait_table; unsigned long wait_table_hash_nr_entries; unsigned long wait_table_bits; /* * Discontig memory support fields. */ struct pglist_data *zone_pgdat; /* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */ unsigned long zone_start_pfn; /* * zone_start_pfn, spanned_pages and present_pages are all * protected by span_seqlock. It is a seqlock because it has * to be read outside of zone->lock, and it is done in the main * allocator path. But, it is written quite infrequently. * * The lock is declared along with zone->lock because it is * frequently read in proximity to zone->lock. It's good to * give them a chance of being in the same cacheline. */ unsigned long spanned_pages; /* total size, including holes */ unsigned long present_pages; /* amount of memory (excluding holes) */ /* * rarely used fields: */ const char *name; } ____cacheline_internodealigned_in_smp;