物理内存中，pglist_data是管理物理内存的最高抽象。
622 typedef struct pglist_data
623 struct zone node_zones[MAX_NR_ZONES];//pglist_data中zone数组
624 struct zonelist node_zonelists[MAX_ZONELISTS];//页面分配的策略，在NUMA中为2，在UMA中为1
625 int nr_zones; //zone的个数
626 #ifdef CONFIG_FLAT_NODE_MEM_MAP /* means !SPARSEMEM */
627 struct page *node_mem_map;
628 #ifdef CONFIG_CGROUP_MEM_RES_CTLR
629 struct page_cgroup *node_page_cgroup;
630 #endif
631 #endif
632 #ifndef CONFIG_NO_BOOTMEM
633 struct bootmem_data *bdata;
634 #endif
635 #ifdef CONFIG_MEMORY_HOTPLUG
636 /*
637 * Must be held any time you expect node_start_pfn, node_present_pages
638 * or node_spanned_pages stay constant. Holding this will also
639 * guarantee that any pfn_valid() stays that way.
640 *
641 * Nests above zone->lock and zone->size_seqlock.
642 */
643 spinlock_t node_size_lock;
644 #endif
645 unsigned long node_start_pfn;//最开始的pfn
646 unsigned long node_present_pages; /* total number of physical pages */
647 unsigned long node_spanned_pages; /* total size of physical page
648 range, including holes */
649 int node_id;
650 wait_queue_head_t kswapd_wait;
651 struct task_struct *kswapd;
652 int kswapd_max_order;
653 } pg_data_t;
在pglist_data下面是zone， 这个结构比较大，但注释很详细。
struct zone_reclaim_stat {
/*
* The pageout code in vmscan.c keeps track of how many of the
* mem/swap backed and file backed pages are refeferenced.
* The higher the rotated/scanned ratio, the more valuable
* that cache is.
*
* The anon LRU stats live in [0], file LRU stats in [1]
*/
unsigned long recent_rotated[2];
unsigned long recent_scanned[2];
/*
* accumulated for batching
*/
unsigned long nr_saved_scan[NR_LRU_LISTS];
};
struct zone {
/* Fields commonly accessed by the page allocator */
/* zone watermarks, access with *_wmark_pages(zone) macros */
unsigned long watermark[NR_WMARK];
/*
* When free pages are below this point, additional steps are taken
* when reading the number of free pages to avoid per-cpu counter
* drift allowing watermarks to be breached
*/
unsigned long percpu_drift_mark;
/*
* 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;
#endif
struct per_cpu_pageset __percpu *pageset;
/*
* free areas of different sizes
*/
spinlock_t lock;
int all_unreclaimable; /* All pages pinned */
#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 */
#ifdef CONFIG_COMPACTION
/*
* On compaction failure, 1< * are skipped before trying again. The number attempted since
* last failure is tracked with compact_considered.
*/
unsigned int compact_considered;
unsigned int compact_defer_shift;
#endif
ZONE_PADDING(_pad1_)
/* Fields commonly accessed by the page reclaim scanner */
spinlock_t lru_lock;
struct zone_lru {
struct list_head list;
} lru[NR_LRU_LISTS];
struct zone_reclaim_stat reclaim_stat;
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()
* invocation.
*
* 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;
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;
自己看吧，遇到要用的时候再回过头来看一下。
最后一个结构是page。
/*
* 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 */
struct page *first_page; /* Compound tail pages */
};
union {
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
};
每个page代表一个物理页面。其中的注释说明了每个变量的作用。
看来学计算机英文很重要，都是英文注释，还好大学的时候好好学了一下英文，一不小心把六级给过了，虽然到现在还是不会说，但认识还是认识的，
面生的可以GOOGLE一下，也就看懂了。
到这你想到了什么？没什么？只是一大堆复制的代码，也没看到什么，但我想到了三层，似乎在计算机这个行业中，三用的挺多的，
比如MVC是三个吧，还有就是Android中的上层用的是java,框架是c++,底层是Linux,当然是C了，自己再去想想吧，
好像别人研究过，层数多余了三便不好让人理解了，不知道是不是这个原因。
物理页面相当于仓库，说明系统中拥有什么，即系统中拥有的资源，跟其它的资源是一样的，只是这里表示的是内存。
为了有效的利用这些资源和安全，系统并没有直接使用这些物理内存，而是使用了虚拟内存，连接虚拟内存与物理内存的是页目录，中间目录，页表，
即y=f(x)其中x为虚拟地址，y是物理地址，不同的x可以对应相同的y。
物理地址有虚拟地址来对应，而设备确有文件来对应，即一切设备皆文件，有某些相同的东西在里面，说不上来，自己想去吧。
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