题外语：本人对linux内核的了解尚浅，如果有差池欢迎指正，也欢迎提问交流！
首先要理解一下每一个进程是如何维护自己独立的寻址空间的，我的电脑里呢是8G内存空间。了解过的朋友应该都知道这是虚拟内存技术解决的这个问题，然而再linux中具体是怎样的模型解决的操作系统的这个设计需求的呢，让我们从linux源码的片段开始看吧！（以下内核源码均来自fedora21 64位系统的fc-3.19.3版本内核）
<include/linux/mm_type.h>中对于物理页面的定义struct page，也就是我们常说的页表，关于这里的结构体的每个变量/位的操作函数大部分在<include/linux/mm.h>中。
1 struct page { 2 /* First double word block */ 3 unsigned long flags; /* Atomic flags, some possibly 4 * updated asynchronously */ 5 union { 6 struct address_space *mapping; /* If low bit clear, points to 7 * inode address_space, or NULL. 8 * If page mapped as anonymous 9 * memory, low bit is set, and 10 * it points to anon_vma object: 11 * see PAGE_MAPPING_ANON below. 12 */ 13 void *s_mem; /* slab first object */ 14 }; 15 16 /* Second double word */ 17 struct { 18 union { 19 pgoff_t index; /* Our offset within mapping. */ 20 void *freelist; /* sl[aou]b first free object */ 21 bool pfmemalloc; /* If set by the page allocator, 22 * ALLOC_NO_WATERMARKS was set 23 * and the low watermark was not 24 * met implying that the system 25 * is under some pressure. The 26 * caller should try ensure 27 * this page is only used to 28 * free other pages. 29 */ 30 }; 31 32 union { 33 #if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \ 34 defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE) 35 /* Used for cmpxchg_double in slub */ 36 unsigned long counters; 37 #else 38 /* 39 * Keep _count separate from slub cmpxchg_double data. 40 * As the rest of the double word is protected by 41 * slab_lock but _count is not. 42 */ 43 unsigned counters; 44 #endif 45 46 struct { 47 48 union { 49 /* 50 * Count of ptes mapped in 51 * mms, to show when page is 52 * mapped & limit reverse map 53 * searches. 54 * 55 * Used also for tail pages 56 * refcounting instead of 57 * _count. Tail pages cannot 58 * be mapped and keeping the 59 * tail page _count zero at 60 * all times guarantees 61 * get_page_unless_zero() will 62 * never succeed on tail 63 * pages. 64 */ 65 atomic_t _mapcount; 66 67 struct { /* SLUB */ 68 unsigned inuse:16; 69 unsigned objects:15; 70 unsigned frozen:1; 71 }; 72 int units; /* SLOB */ 73 }; 74 atomic_t _count; /* Usage count, see below. */ 75 }; 76 unsigned int active; /* SLAB */ 77 }; 78 }; 79 80 /* Third double word block */ 81 union { 82 struct list_head lru; /* Pageout list, eg. active_list 83 * protected by zone->lru_lock ! 84 * Can be used as a generic list 85 * by the page owner. 86 */ 87 struct { /* slub per cpu partial pages */ 88 struct page *next; /* Next partial slab */ 89 #ifdef CONFIG_64BIT 90 int pages; /* Nr of partial slabs left */ 91 int pobjects; /* Approximate # of objects */ 92 #else 93 short int pages; 94 short int pobjects; 95 #endif 96 }; 97 98 struct slab *slab_page; /* slab fields */ 99 struct rcu_head rcu_head; /* Used by SLAB 100 * when destroying via RCU 101 */ 102 #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS 103 pgtable_t pmd_huge_pte; /* protected by page->ptl */ 104 #endif 105 }; 106 107 /* Remainder is not double word aligned */ 108 union { 109 unsigned long private; /* Mapping-private opaque data: 110 * usually used for buffer_heads 111 * if PagePrivate set; used for 112 * swp_entry_t if PageSwapCache; 113 * indicates order in the buddy 114 * system if PG_buddy is set. 115 */ 116 #if USE_SPLIT_PTE_PTLOCKS 117 #if ALLOC_SPLIT_PTLOCKS 118 spinlock_t *ptl; 119 #else 120 spinlock_t ptl; 121 #endif 122 #endif 123 struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */ 124 struct page *first_page; /* Compound tail pages */ 125 }; 126 127 #ifdef CONFIG_MEMCG 128 struct mem_cgroup *mem_cgroup; 129 #endif 130 131 /* 132 * On machines where all RAM is mapped into kernel address space, 133 * we can simply calculate the virtual address. On machines with 134 * highmem some memory is mapped into kernel virtual memory 135 * dynamically, so we need a place to store that address. 136 * Note that this field could be 16 bits on x86 ... ;) 137 * 138 * Architectures with slow multiplication can define 139 * WANT_PAGE_VIRTUAL in asm/page.h 140 */ 141 #if defined(WANT_PAGE_VIRTUAL) 142 void *virtual; /* Kernel virtual address (NULL if 143 not kmapped, ie. highmem) */ 144 #endif /* WANT_PAGE_VIRTUAL */ 145 146 #ifdef CONFIG_KMEMCHECK 147 /* 148 * kmemcheck wants to track the status of each byte in a page; this 149 * is a pointer to such a status block. NULL if not tracked. 150 */ 151 void *shadow; 152 #endif 153 154 #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS 155 int _last_cpupid; 156 #endif 157 }