swapper_pg_dir的作用

千金一刻莫空度，老大无成空自伤。这篇文章主要讲述swapper_pg_dir的作用相关的知识，希望能为你提供帮助。
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));

【swapper_pg_dir的作用】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。