在内存系统初始化过程中，有如下代码：
1: static void __init pagetable_init(void)
2: {
3: pgd_t *pgd_base = swapper_pg_dir;
4:
5: permanent_kmaps_init(pgd_base);
6: }
这里，我们看到了神秘的swapper_pg_dir，全局搜索一下，发现了
1: /*
2: * Build a proper pagetable for the kernel mappings. Up until this
3: * point, we‘ve been running on some set of pagetables constructed by
4: * the boot process.
5: *
6: * If we‘re booting on native hardware, this will be a pagetable
7: * constructed in arch/x86/kernel/head_32.S. The root of the
8: * pagetable will be swapper_pg_dir.
9: *
10: * If we‘re booting paravirtualized under a hypervisor, then there are
11: * more options: we may already be running PAE, and the pagetable may
12: * or may not be based in swapper_pg_dir. In any case,
13: * paravirt_pagetable_setup_start() will set up swapper_pg_dir
14: * appropriately for the rest of the initialization to work.
15: *
16: * In general, pagetable_init() assumes that the pagetable may already
17: * be partially populated, and so it avoids stomping on any existing
18: * mappings.
19: */
20: void __init early_ioremap_page_table_range_init(void)
21: {
22: pgd_t *pgd_base = swapper_pg_dir;
23: unsigned long vaddr, end;
24:
25: /*
26: * Fixed mappings, only the page table structure has to be
27: * created - mappings will be set by set_fixmap():
28: */
29: vaddr = __fix_to_virt(__end_of_fixed_addresses - 1) & PMD_MASK;
30: end = (FIXADDR_TOP + PMD_SIZE - 1) & PMD_MASK;
31: page_table_range_init(vaddr, end, pgd_base);
32: early_ioremap_reset();
33: }
在head_32.S中，定义了如下的BSS段，BSS段是在内核映像文件中不占空间，但是在内核被加载到内存时，会保留相应的空间。
在BSS段，一共保留了4个页面的空间，分别用initial_page_table, initial_pg_fixmap, empty_zero_page和swapper_pg_dir来标志其地址。
1: /*
2: * BSS section
3: */
4: __PAGE_ALIGNED_BSS
5: .align PAGE_SIZE
6: #ifdef CONFIG_X86_PAE
7: initial_pg_pmd:
8: .fill 1024*KPMDS,4,0
9: #else
10: ENTRY(initial_page_table)
11: .fill 1024,4,0
12: #endif
13: initial_pg_fixmap:
14: .fill 1024,4,0
15: ENTRY(empty_zero_page)
16: .fill 4096,1,0
17: ENTRY(swapper_pg_dir)
18: .fill 1024,4,0
通过如下代码，将initial_page_table设置为初始页目录
1: /*
2: * Enable paging
3: */
4: movl $pa(initial_page_table), %eax
5: movl %eax,%cr3 /* set the page table pointer.. */
6: movl %cr0,%eax
7: orl $X86_CR0_PG,%eax
8: movl %eax,%cr0 /* ..and set paging (PG) bit */
9: ljmp $__BOOT_CS,$1f /* Clear prefetch and normalize %eip */
在内核初始化阶段，setup_arch调用了如下的函数：
1: void __init setup_arch(char **cmdline_p)
2: {
3:
4: ......
5: /* max_pfn_mapped is updated here */
6: max_low_pfn_mapped = init_memory_mapping(0, max_low_pfn<<PAGE_SHIFT);
7: max_pfn_mapped = max_low_pfn_mapped;
8: ......
9: x86_init.paging.pagetable_setup_start(swapper_pg_dir);
10: paging_init();
11: x86_init.paging.pagetable_setup_done(swapper_pg_dir);
12:
13: ......
14: }
init_memory_mapping调用了kernel_physical_mapping_init，初始化swapper_pg_dir
1: /*
2: * This maps the physical memory to kernel virtual address space, a total
3: * of max_low_pfn pages, by creating page tables starting from address
4: * PAGE_OFFSET:
5: */
6: unsigned long __init
7: kernel_physical_mapping_init(unsigned long start,
8: unsigned long end,
9: unsigned long page_size_mask)
10: {
11: int use_pse = page_size_mask == (1<<PG_LEVEL_2M);
12: unsigned long last_map_addr = end;
13: unsigned long start_pfn, end_pfn;
14: pgd_t *pgd_base = swapper_pg_dir;
15: int pgd_idx, pmd_idx, pte_ofs;
16: unsigned long pfn;
17: pgd_t *pgd;
18: pmd_t *pmd;
19: pte_t *pte;
20: unsigned pages_2m, pages_4k;
21: int mapping_iter;
22:
23: start_pfn = start >> PAGE_SHIFT;
24: end_pfn = end >> PAGE_SHIFT;
25:
26: /*
27: * First iteration will setup identity mapping using large/small pages
28: * based on use_pse, with other attributes same as set by
29: * the early code in head_32.S
30: *
31: * Second iteration will setup the appropriate attributes (NX, GLOBAL..)
32: * as desired for the kernel identity mapping.
33: *
34: * This two pass mechanism conforms to the TLB app note which says:
35: *
36: * "Software should not write to a paging-structure entry in a way
37: * that would change, for any linear address, both the page size
38: * and either the page frame or attributes."
39: */
40: mapping_iter = 1;
41:
42: if (!cpu_has_pse)
43: use_pse = 0;
44:
45: at:
46: pages_2m = pages_4k = 0;
47: pfn = start_pfn;
48: pgd_idx = pgd_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET);
49: pgd = pgd_base + pgd_idx;
50: for (; pgd_idx < PTRS_PER_PGD; pgd++, pgd_idx++) {
51: pmd = one_md_table_init(pgd);
52:
53: if (pfn >= end_pfn)
54: continue;
55: ef CONFIG_X86_PAE
56: pmd_idx = pmd_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET);
57: pmd += pmd_idx;
58: e
59: pmd_idx = 0;
60: if
61: for (; pmd_idx < PTRS_PER_PMD && pfn < end_pfn;
62: pmd++, pmd_idx++) {
63: unsigned int addr = pfn * PAGE_SIZE + PAGE_OFFSET;
64:
65: /*
66: * Map with big pages if possible, otherwise
67: * create normal page tables:
68: */
69: if (use_pse) {
70: unsigned int addr2;
71: pgprot_t prot = PAGE_KERNEL_LARGE;
72: /*
73: * first pass will use the same initial
74: * identity mapping attribute + _PAGE_PSE.
75: */
76: pgprot_t init_prot =
77: __pgprot(PTE_IDENT_ATTR |
78: _PAGE_PSE);
79:
80: addr2 = (pfn + PTRS_PER_PTE-1) * PAGE_SIZE +
81: PAGE_OFFSET + PAGE_SIZE-1;
82:
83: if (is_kernel_text(addr) ||
84: is_kernel_text(addr2))
85: prot = PAGE_KERNEL_LARGE_EXEC;
86:
87: pages_2m++;
88: if (mapping_iter == 1)
89: set_pmd(pmd, pfn_pmd(pfn, init_prot));
90: else
91: set_pmd(pmd, pfn_pmd(pfn, prot));
92:
93: pfn += PTRS_PER_PTE;
94: continue;
95: }
96: pte = one_page_table_init(pmd);
97:
98: pte_ofs = pte_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET);
99: pte += pte_ofs;
100: for (; pte_ofs < PTRS_PER_PTE && pfn < end_pfn;
101: pte++, pfn++, pte_ofs++, addr += PAGE_SIZE) {
102: pgprot_t prot = PAGE_KERNEL;
103: /*
104: * first pass will use the same initial
105: * identity mapping attribute.
106: */
107: pgprot_t init_prot = __pgprot(PTE_IDENT_ATTR);
108:
109: if (is_kernel_text(addr))
110: prot = PAGE_KERNEL_EXEC;
111:
112: pages_4k++;
113: if (mapping_iter == 1) {
114: set_pte(pte, pfn_pte(pfn, init_prot));
115: last_map_addr = (pfn << PAGE_SHIFT) + PAGE_SIZE;
116: } else
117: set_pte(pte, pfn_pte(pfn, prot));
118: }
119: }
120: }
121: if (mapping_iter == 1) {
122: /*
123: * update direct mapping page count only in the first
124: * iteration.
125: */
126: update_page_count(PG_LEVEL_2M, pages_2m);
127: update_page_count(PG_LEVEL_4K, pages_4k);
128:
129: /*
130: * local global flush tlb, which will flush the previous
131: * mappings present in both small and large page TLB‘s.
132: */
133: __flush_tlb_all();
134:
135: /*
136: * Second iteration will set the actual desired PTE attributes.
137: */
138: mapping_iter = 2;
139: goto repeat;
140: }
141: return last_map_addr;
142: }
pgd_t *pgd_base = swapper_pg_dir;
将swapper_pg_dir作为页目录地址，赋给pgd_base
start_pfn = start >> PAGE_SHIFT;
end_pfn = end >> PAGE_SHIFT;
pgd_idx = pgd_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET);
pgd = pgd_base + pgd_idx;
pgd_idx,pgd代表着在页目录中的索引，以及相应的页目录项
1: pgprot_t prot = PAGE_KERNEL;
2: /*
3: * first pass will use the same initial
4: * identity mapping attribute.
5: */
6: pgprot_t init_prot = __pgprot(PTE_IDENT_ATTR);
7:
8: if (is_kernel_text(addr))
9: prot = PAGE_KERNEL_EXEC;
10:
11: pages_4k++;
12: if (mapping_iter == 1) {
13: set_pte(pte, pfn_pte(pfn, init_prot));
14: last_map_addr = (pfn << PAGE_SHIFT) + PAGE_SIZE;
15: } else
16: set_pte(pte, pfn_pte(pfn, prot));
最后，通过两个回合的遍历，将属性设置到对应的页表项上去。