LINUX内核地址空间的布局

1、linux内核地址空间的布局1) linux将整个4g线性地址空间分为用户空间和内核空间两部分,而内核地址空间又被划分为物理内存区, 虚拟内存分配区, 高端页面映射区,专用页面映射区, 系统保留映射区几个区域. 2) 在标准配置下, 物理区最大长度为896m, 系统的物理内存被顺序映射在物理区中,在支持扩展页长(pse)和全局页面(pge)的机器上, 物理区使用4m页面并作为全局页面来处理.当系统物理内存大于896m时, 超过物理区的那部分内存称为高端内存,低端内存和高端内存用highmem_start_page变量来定界,内核在存取高端内存时必须将它们映射到高端页面映射区. 3) linux

2、保留内核空间最顶部128k区域作为保留区,紧接保留区以下的一段区域为专用页面映射区,它的总尺寸和每一页的用途由fixed_address枚举结构在编绎时预定义,用_fix_to_virt(index)可获取专用区内预定义页面的逻辑地址.在专用页面区内为每个cpu预定义了一张高端内存映射页, 用于在中断处理中高端页面的映射操作.4) 距离内核空间顶部32m, 长度为4m的一段区域为高端内存映射区,它正好占用1个页帧表所表示的物理内存总量, 它可以缓冲1024个高端页面的映射.在物理区和高端映射区之间为虚存内存分配区, 用于vmalloc()函数,它的前部与物理区有8m隔离带, 后部与高端映射区有

3、8k的隔离带. 5) 当系统物理内存超过4g时,必须使用cpu的扩展分页(pae)模式所提供的64位页目录项才能存取到4g以上的物理内存.在pae模式下, 线性地址到物理地址的转换使用3级页表, 第1级页目录由线性地址的最高2位索引,每一目录项对应1g的寻址空间, 第2级页目录项以9位索引, 每一目录项对应2m的寻址空间,第3级页目录项以9位索引, 每一目录项对应4k的页帧.除了页目录项所描述的物理地址扩展为36位外, 64位和32位页目录项结构没有什么区别.在pae模式下, 包含pse位的中级页目录项所对应的页面从4m减少为2m. /* just any arbitrary offset t

4、o the start of the vmalloc vm area: the * current 8mb value just means that there will be a 8mb hole after the * physical memory until the kernel virtual memory starts. that means that * any out-of-bounds memory accesses will hopefully be caught. * the vmalloc() routines leaves a hole of 4kb between

5、 each vmalloced * area for the same reason. ;) */#define vmalloc_offset(8*1024*1024) 虚拟区与物理区的隔离宽度#define vmalloc_start(unsigned long) high_memory + 2*vmalloc_offset-1) & (vmalloc_offset-1) 虚拟区的开始#define vmalloc_vmaddr(x) (unsigned long)(x)#if config_highmem# define vmalloc_end(pkmap_base-2*page_size

6、)#else# define vmalloc_end(fixaddr_start-2*page_size)#endif#define pkmap_base (0xfe000000ul) 高端映射区的开始#define fixaddr_top(0xffffe000ul) 专用映射区的顶部#define fixaddr_size(_end_of_fixed_addresses page_shift) 专用映射区尺寸#define fixaddr_start(fixaddr_top - fixaddr_size) 专用映射区开始#define _fix_to_virt(x)(fixaddr_top

7、- (x) page_shift) 取专用映射区页面/* * on up currently we will have no trace of the fixmap mechanizm, * no page table allocations, etc. this might change in the * future, say framebuffers for the console driver(s) could be * fix-mapped? */enum fixed_addresses 专用区页面的功能定义#ifdef config_x86_local_apicfix_apic_b

8、ase,/* local (cpu) apic) - required for smp or not */#endif#ifdef config_x86_io_apicfix_io_apic_base_0,fix_io_apic_base_end = fix_io_apic_base_0 + max_io_apics-1,#endif#ifdef config_x86_visws_apicfix_co_cpu,/* cobalt timer */fix_co_apic,/* cobalt apic redirection table */ fix_li_pcia,/* lithium pci

9、bridge a */fix_li_pcib,/* lithium pci bridge b */#endif#ifdef config_highmem 用于中断处理内映射高端页面fix_kmap_begin,/* reserved ptes for temporary kernel mappings */fix_kmap_end = fix_kmap_begin+(km_type_nr*nr_cpus)-1,#endif_end_of_fixed_addresses;enum km_type km_bounce_read,km_bounce_write,km_type_nr; arch/i3

10、86/mm/init.c:static void _init pagetable_init (void) 建立内核空间的页表unsigned long vaddr, end;pgd_t *pgd, *pgd_base;int i, j, k;pmd_t *pmd;pte_t *pte, *pte_base;/* * this can be zero as well - no problem, in that case we exit * the loops anyway due to the ptrs_per_* conditions. */end = (unsigned long)_va(m

11、ax_low_pfn*page_size); 取内核物理内存区域终止边界物理地址pgd_base = swapper_pg_dir; 取内核基准页目录表地址#if config_x86_pae 在pae模式下, 首级页表只包含4个64位目录项for (i = 0; i ptrs_per_pgd; i+) pgd = pgd_base + i;_pgd_clear(pgd);#endifi = _pgd_offset(page_offset); 取内核空间起始边界在首级页表内的地址偏移pgd = pgd_base + i; for (; i = end) 如果该映射区域起始边界大于或等于内核物理

12、内存的终止边界break; 退出扫描#if config_x86_paepmd = (pmd_t *) alloc_bootmem_low_pages(page_size); 分配中级页表set_pgd(pgd, _pgd(_pa(pmd) + 0x1); 将中级页表登记到首级页表#elsepmd = (pmd_t *)pgd; #endifif (pmd != pmd_offset(pgd, 0)bug();for (j = 0; j = end)break;if (cpu_has_pse) 如果cpu具有扩展页长功能unsigned long _pe;set_in_cr4(x86_cr4_

13、pse);boot_cpu_data.wp_works_ok = 1; _pe = _kernpg_table + _page_pse + _page_user + _pa(vaddr); 使用扩展页长目录项/* make it global too if supported */if (cpu_has_pge) set_in_cr4(x86_cr4_pge);_pe += _page_global; 设置全局页目录项标志set_pmd(pmd, _pmd(_pe); 设置中级页表continue; 继续下一中级页目录项; 不使用扩展页长pte_base = pte = (pte_t *) a

14、lloc_bootmem_low_pages(page_size); 分配页帧表for (k = 0; k = end)break;*pte = mk_pte_phys(_pa(vaddr), page_kernel); 设置页帧表set_pmd(pmd, _pmd(_kernpg_table + _pa(pte_base); 设置中级页表if (pte_base != pte_offset(pmd, 0)bug();/* * fixed mappings, only the page table structure has to be * created - mappings will be

15、 set by set_fixmap(): */vaddr = _fix_to_virt(_end_of_fixed_addresses - 1) & pmd_mask;取专用区起始地址所在的中级页目录边界地址fixrange_init(vaddr, 0, pgd_base); 建立专用区页表#if config_highmem/* * permanent kmaps: */vaddr = pkmap_base; fixrange_init(vaddr, vaddr + page_size*last_pkmap, pgd_base); 建立高端映射区页表pgd = swapper_pg_dir

16、 + _pgd_offset(vaddr);pmd = pmd_offset(pgd, vaddr);pte = pte_offset(pmd, vaddr);pkmap_page_table = pte; 设置高端映射区所在的页帧目录#endif#if config_x86_pae/* * add low memory identity-mappings - smp needs it when * starting up on an ap from real-mode. in the non-pae * case we already have these mappings through

17、head.s. * all user-space mappings are explicitly cleared after * smp startup. */pgd_base0 = pgd_baseuser_ptrs_per_pgd;#endifstatic void _init fixrange_init (unsigned long start, unsigned long end, pgd_t*pgd_base)pgd_t *pgd;pmd_t *pmd;pte_t *pte;int i, j;unsigned long vaddr;vaddr = start;i = _pgd_offset(vaddr);j = _pmd_offset(vaddr);pgd = pgd_base + i;for ( ; (i ptrs_per_pgd) & (vaddr != end); pgd+, i+) #if config_x86_paeif (pgd_none(*pgd) pmd = (pmd_t *) alloc_bootmem_low_pages(page_size);set_pgd(pgd, _pgd(_pa(pmd) + 0x1);if (pmd != pmd_offset(pgd, 0)printk(pae bug #02!n);