swapper_pg_dir的作用

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<

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<> 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<= end_pfn)
  54:             continue;
  55:     ef CONFIG_X86_PAE
  56:         pmd_idx = pmd_index((pfn<

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<
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。