linux使用于广泛的体系结构，因此需要用一种与体系结构无关的方式来描述内存。linux用VM描述和管理内存。在VM中兽药的普遍概念就是非一致内存访问。对于大型机器而言，内存会分成许多簇，依据簇与处理器“距离”的不同，访问不同的簇会有不同的代价。
每个簇都被认为是一个节点（pg_data_t)，每个节点被分成很多的成为管理区（zone)的块，用于表示内存中的某个范围。除了ZONE_DMA,ZONE_NORMAL,ZONE_HIGHMEM以外，linux2.6.32中引入了ZONE_MOVABLE，用于适应大块连续内存的分配。
每个物理页面由一个page结构体描述，所有的结构都存储在一个全局的mem_map数组中（非平板模式），该数组通常存放在ZONE_NORMAL的首部，或者就在校内存系统中为装入内核映像而预留的区域之后。
节点
内存的每个节点都有pg_data_t描述，在分配一个页面时，linux采用节点局部分配的策略，从最靠近运行中的CPU的节点分配内存。由于进程往往是在同一个CPU上运行，因此从当前节点得到的内存很可能被用到。
/*
* 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 {
/*该节点内的内存区。可能的区域类型用zone_type表示。 */
struct zone node_zones[MAX_NR_ZONES];
/* 该节点的备用内存区。当节点没有可用内存时，就从备用区中分配内存。*/
struct zonelist node_zonelists[MAX_ZONELISTS];
/*可用内存区数目，即node_zones数据中保存的最后一个有效区域的索引*/
int nr_zones;
#ifdef CONFIG_FLAT_NODE_MEM_MAP /* means !SPARSEMEM */
/* 在平坦型的内存模型中，它指向本节点第一个页面的描述符。 */
struct page *node_mem_map;
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
/*cgroup相关*/
struct page_cgroup *node_page_cgroup;
#endif
#endif
/**
* 在内存子系统初始化以前，即boot阶段也需要进行内存管理。
* 此结构用于这个阶段的内存管理。
*/
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.
*/
/*当系统支持内存热插拨时，用于保护本结构中的与节点大小相关的字段。
哪调用node_start_pfn，node_present_pages，node_spanned_pages相关的代码时，需要使用该锁。
*/
spinlock_t node_size_lock;
#endif
/*起始页面帧号，指出该节点在全局mem_map中
的偏移*/
unsigned long node_start_pfn;
unsigned long node_present_pages; /* total number of physical pages */
unsigned long node_spanned_pages; /* total size of physical page range, including holes */
/*节点编号*/
int node_id;
/*等待该节点内的交换守护进程的等待队列。将节点中的页帧换出时会用到。*/
wait_queue_head_t kswapd_wait;
/*负责该节点的交换守护进程。*/
struct task_struct *kswapd;
/*由页交换子系统使用，定义要释放的区域大小。*/
int kswapd_max_order;
} pg_data_t;
管理区
每个管理区由一个zone结构体描述，对于管理区的类型描述如下
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
/*
这是一个伪内存段。为了防止形成物理内存碎片，
可以将虚拟地址对应的物理地址进行迁移。
*/
ZONE_MOVABLE,
__MAX_NR_ZONES
};
里面的英文注释已经写的很详细了。
管理区用于跟踪诸如页面使用情况统计数，空闲区域信息和锁信息等。
struct zone {
/* Fields commonly accessed by the page allocator */
/* zone watermarks, access with *_wmark_pages(zone) macros */
/*本管理区的三个水线值:高水线(比较充足)、低水线、MIN水线。*/
unsigned long watermark[NR_WMARK];
/*
* 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.
*/
/**
* 当高端内存、normal内存区域中无法分配到内存时，需要从normal、DMA区域中分配内存。
* 为了避免DMA区域被消耗光，需要额外保留一些内存供驱动使用。
* 该字段就是指从上级内存区退到回内存区时，需要额外保留的内存数量。
*/
unsigned long lowmem_reserve[MAX_NR_ZONES];
#ifdef CONFIG_NUMA
/*所属的NUMA节点。*/
int node;
/*
* zone reclaim becomes active if more unmapped pages exist.
*/
/*当可回收的页超过此值时，将进行页面回收。*/
unsigned long min_unmapped_pages;
/*当管理区中，用于slab的可回收页大于此值时，将回收slab中的缓存页。*/
unsigned long min_slab_pages;
/*
* 每CPU的页面缓存。
* 当分配单个页面时，首先从该缓存中分配页面。这样可以:
*避免使用全局的锁
* 避免同一个页面反复被不同的CPU分配，引起缓存行的失效。
* 避免将管理区中的大块分割成碎片。
*/
struct per_cpu_pageset *pageset[NR_CPUS];
#else
struct per_cpu_pageset pageset[NR_CPUS];
#endif
/*
* free areas of different sizes
*/
/*该锁用于保护伙伴系统数据结构。即保护free_area相关数据。*/
spinlock_t lock;
#ifdef CONFIG_MEMORY_HOTPLUG
/* see spanned/present_pages for more description */
/*用于保护spanned/present_pages等变量。这些变量几乎不会发生变化，除非发生了内存热插拨操作。
这几个变量并不被lock字段保护。并且主要用于读，因此使用读写锁。*/
seqlock_t span_seqlock;
#endif
/*伙伴系统的主要变量。这个数组定义了11个队列，每个队列中的元素都是大小为2^n的页面*/
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
*/
/*本管理区里的页面标志数组*/
unsigned long *pageblock_flags;
#endif /* CONFIG_SPARSEMEM */
/*填充的未用字段，确保后面的字段是缓存行对齐的*/
ZONE_PADDING(_pad1_)
/* Fields commonly accessed by the page reclaim scanner */
/*
* lru相关的字段用于内存回收。这个字段用于保护这几个回收相关的字段。
* lru用于确定哪些字段是活跃的，哪些不是活跃的，并据此确定应当被写回到磁盘以释放内存。
*/
spinlock_t lru_lock;
/* 匿名活动页、匿名不活动页、文件活动页、文件不活动页链表头*/
struct zone_lru {
struct list_head list;
} lru[NR_LRU_LISTS];
/*页面回收状态*/
struct zone_reclaim_stat reclaim_stat;
/*自从最后一次回收页面以来，扫过的页面数*/
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;
/*
* The target ratio of ACTIVE_ANON to INACTIVE_ANON pages on
* this zone's LRU. Maintained by the pageout code.
*/
unsigned int inactive_ratio;
/*为cache对齐*/
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.
*/
/*管理区属于的节点*/
struct pglist_data *zone_pgdat;
/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
/*管理区的页面在mem_map中的偏移*/
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;
没有说明的地方，内核中的英文注释已经写得很清楚了。
页面
系统中每个物理页面都有一个相关联的page用于记录该页面的状态。
/*
* Each physical page in the system has a struct page associated with
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
*/
struct page {
unsigned long flags; /* Atomic flags, some possibly
* updated asynchronously */
atomic_t _count; /* Usage count, see below. */
union {
atomic_t _mapcount; /* Count of ptes mapped in mms,
* to show when page is mapped
* & limit reverse map searches.
*/
struct { /* SLUB */
u16 inuse;
u16 objects;
};
};
union {
struct {
unsigned long private; /* Mapping-private opaque data:
* usually used for buffer_heads
* if PagePrivate set; used for
* swp_entry_t if PageSwapCache;
* indicates order in the buddy
* system if PG_buddy is set.
*/
struct address_space *mapping; /* If low bit clear, points to
* inode address_space, or NULL.
* If page mapped as anonymous
* memory, low bit is set, and
* it points to anon_vma object:
* see PAGE_MAPPING_ANON below.
*/
};
#if USE_SPLIT_PTLOCKS
spinlock_t ptl;
#endif
struct kmem_cache *slab; /* SLUB: Pointer to slab */
/* 如果属于伙伴系统，并且不是伙伴系统中的第一个页
则指向第一个页*/
struct page *first_page; /* Compound tail pages */
};
union {/*如果是文件映射，那么表示本页面在文件中的位置(偏移)*/
pgoff_t index; /* Our offset within mapping. */
void *freelist; /* SLUB: freelist req. slab lock */
};
struct list_head lru; /* Pageout list, eg. active_list
* protected by zone->lru_lock !
*/
/*
* On machines where all RAM is mapped into kernel address space,
* we can simply calculate the virtual address. On machines with
* highmem some memory is mapped into kernel virtual memory
* dynamically, so we need a place to store that address.
* Note that this field could be 16 bits on x86 ... ;)
*
* Architectures with slow multiplication can define
* WANT_PAGE_VIRTUAL in asm/page.h
*/
#if defined(WANT_PAGE_VIRTUAL)
void *virtual; /* Kernel virtual address (NULL if
not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
unsigned long debug_flags; /* Use atomic bitops on this */
#endif
#ifdef CONFIG_KMEMCHECK
/*
* kmemcheck wants to track the status of each byte in a page; this
* is a pointer to such a status block. NULL if not tracked.
*/
void *shadow;
#endif
};
linux中主要的结构描述体现了linux物理内存管理的设计。后面会介绍linux内存管理的各个细节。