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Linux內(nèi)核高-低端內(nèi)存設(shè)置代碼跟蹤（ARM構(gòu)架）

作者： 時(shí)間：2016-11-21 來源：網(wǎng)絡(luò) 收藏
對于ARM中內(nèi)核如何在啟動(dòng)的時(shí)候設(shè)置高低端內(nèi)存的分界線（也是邏輯地址與虛擬地址分界線（虛擬地址）減去那個(gè)固定的偏移），這里我稍微引導(dǎo)下（內(nèi)核分析使用Linux-3.0）：
首先定位設(shè)置內(nèi)核虛擬地址起始位置（也就是內(nèi)核邏輯地址末端+1的地址）的文件：init.c (archarmmm)，在這個(gè)文件中的void __init bootmem_init(void)函數(shù)如下
  1. void __init bootmem_init(void)
  2. {
  3. unsigned long min,max_low,max_high;
  5. max_low=max_high=0;
  7. find_limits(&min,&max_low,&max_high);
  9. arm_bootmem_init(min,max_low);
  11. /*
  12. *Sparsemem triestoallocate bootmeminmemory_present(),
  13. *so must be done after the fixed reservations
  14. */
  15. arm_memory_present();
  17. /*
  18. *sparse_init()needs the bootmem allocator upandrunning.
  19. */
  20. sparse_init();
  22. /*
  23. *Nowfree the memory-free_area_init_node needs
  24. *the sparse mem_map arrays initialized by sparse_init()
  25. *formemmap_init_zone(),otherwise all PFNs are invalid.
  26. */
  27. arm_bootmem_free(min,max_low,max_high);
  29. high_memory = __va(((phys_addr_t)max_low << PAGE_SHIFT) - 1) + 1;
  31. /*
  32. *This doesnt seemtobe used by the Linux memory manager any
  33. *more,butisused by ll_rw_block.Ifwe cangetrid of it,we
  34. *alsogetrid of some of the stuff above as well.
  35. *
  36. *Note:max_low_pfnandmax_pfn reflect the number of _pages_in
  37. *the system,notthe maximum PFN.
  38. */
  39. max_low_pfn=max_low-PHYS_PFN_OFFSET;
  40. max_pfn=max_high-PHYS_PFN_OFFSET;
  41. }
這個(gè)high_memory=__va(((phys_addr_t)max_low<<PAGE_SHIFT)-1)+1;語句就是關(guān)鍵。從這里可以知道m(xù)ax_low就是高端內(nèi)存的起始地址（物理地址）。那么這個(gè)max_low是如何得到的？其實(shí)看上面的代碼可以推測出，他其實(shí)是在find_limits(&min,&max_low,&max_high);中（在同一個(gè)文件中）被設(shè)置的：
  1. static void __init find_limits(unsigned long*min,unsigned long*max_low,
  2. unsigned long*max_high)
  3. {
  4. struct meminfo*mi=&meminfo;
  5. inti;
  7. *min=-1UL;
  8. *max_low=*max_high=0;
  10. for_each_bank(i,mi){
  11. struct membank*bank=&mi->bank[i];
  12. unsigned long start,end;
  14. start=bank_pfn_start(bank);
  15. end=bank_pfn_end(bank);
  17. if(*min>start)
  18. *min=start;
  19. if(*max_high<end)
  20. *max_high=end;
  21. if (bank->highmem)
  22. continue;
  23. if (*max_low < end)
  24. *max_low = end;
  25. }
  26. }
這個(gè)函數(shù)的意思很明顯：通過掃描struct meminfo*mi=&meminfo;（結(jié)構(gòu)體meminfo的數(shù)組）中的所有信息，設(shè)置三個(gè)指針?biāo)傅淖兞浚?/div>
  1. min :內(nèi)存物理地址起始
  2. max_low :低端內(nèi)存區(qū)物理地址末端
  3. max_high :高端內(nèi)存區(qū)物理地址末端
從上面可以看出，max_low和max_high所保存的地址不同就是由于bank->highmem造成的，它是內(nèi)存bank被設(shè)為高端內(nèi)存的依據(jù)：
  1. “如果這個(gè)內(nèi)存bank是高端內(nèi)存（bank->highmem ！= 0），跳過max_low = end;語句，max_low和max_high將不同（結(jié)果實(shí)際上是max_high >max_low）；
  2. 否則假設(shè)沒有一個(gè)內(nèi)存bank是高端內(nèi)存（所有bank->highmem == 0）max_low和max_high必然一致（高端內(nèi)存大小為0）”
當(dāng)然要實(shí)現(xiàn)這個(gè)函數(shù)的功能，必須保證meminfo所指數(shù)組中的所有bank是按照地址數(shù)據(jù)從小到大排序好的哦～～。但是這個(gè)大家不用擔(dān)心，后面會(huì)看到的：）
經(jīng)過上面的跟蹤，焦點(diǎn)集中到了全局變量（同一個(gè)文件中）：
  1. struct meminfo meminfo;
這個(gè)結(jié)構(gòu)體的定義（setup.h (archarmincludeasm)）：
  1. /*
  2. * Memory map description
  3. */
  4. #define NR_BANKS 8
  6. struct membank {
  7. phys_addr_t start;
  8. unsigned long size;
  9. unsigned int highmem;
  10. };
  12. struct meminfo {
  13. int nr_banks;
  14. struct membank bank[NR_BANKS];
  15. };
  17. extern struct meminfo meminfo;
  19. #define for_each_bank(iter,mi)
  20. for (iter = 0; iter < (mi)->nr_banks; iter++)
#define bank_pfn_start(bank) __phys_to_pfn((bank)->start)
#define bank_pfn_end(bank) __phys_to_pfn((bank)->start + (bank)->size)
#define bank_pfn_size(bank) ((bank)->size >> PAGE_SHIFT)
#define bank_phys_start(bank) (bank)->start
#define bank_phys_end(bank) ((bank)->start + (bank)->size)
#define bank_phys_size(bank) (bank)->size
只要找到初始化這個(gè)全局變量并完成排序的地方，就可以知道高端內(nèi)存是如何配置的了??！OK，明確目標(biāo)，go on～～～
通過查找代碼，我們可以在setup.c (archarmkernel)這個(gè)文件中找到相關(guān)的代碼。在系統(tǒng)啟動(dòng)早期會(huì)運(yùn)行的函數(shù)（具體的順序你可以自行分析下ARM內(nèi)核的啟動(dòng)流程，以后我也會(huì)寫下）中有這樣一個(gè)函數(shù)：
  1. void __init setup_arch(char **cmdline_p)
  2. {
  3. struct machine_desc *mdesc;
  5. unwind_init();
  7. setup_processor();
  8. mdesc = setup_machine_fdt(__atags_pointer);
  9. if (!mdesc)
  10. mdesc = setup_machine_tags(machine_arch_type);
  11. machine_desc = mdesc;
  12. machine_name = mdesc->name;
  14. if (mdesc->soft_reboot)
  15. reboot_setup("s");
  17. init_mm.start_code = (unsigned long) _text;
  18. init_mm.end_code = (unsigned long) _etext;
  19. init_mm.end_data = (unsigned long) _edata;
  20. init_mm.brk = (unsigned long) _end;
  23. strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
  24. *cmdline_p = cmd_line;
  26. parse_early_param();
  28. sanity_check_meminfo();
  29. arm_memblock_init(&meminfo, mdesc);
  31. paging_init(mdesc);
  32. request_standard_resources(mdesc);
  34. unflatten_device_tree();
  36. #ifdef CONFIG_SMP
  37. if (is_smp())
  38. smp_init_cpus();
  39. #endif
  40. reserve_crashkernel();
  42. cpu_init();
  43. tcm_init();
  45. #ifdef CONFIG_MULTI_IRQ_HANDLER
  46. handle_arch_irq = mdesc->handle_irq;
  47. #endif
  49. #ifdef CONFIG_VT
  50. #if defined(CONFIG_VGA_CONSOLE)
  51. conswitchp = &vga_con;
  52. #elif defined(CONFIG_DUMMY_CONSOLE)
  53. conswitchp = &dummy_con;
  54. #endif
  55. #endif
  56. early_trap_init();
  58. if (mdesc->init_early)
  59. mdesc->init_early();
  60. }
在上面的注釋中，我已經(jīng)表明了重點(diǎn)和解析，下面我細(xì)化下：
（1）獲取參數(shù)部分
通過parse_early_param();函數(shù)可以解析內(nèi)核啟動(dòng)參數(shù)中的許多字符串，但是對于我們這次分析內(nèi)存的話主要是分析以下兩個(gè)參數(shù)：
mem=size@start參數(shù)，她為初始化struct meminfo meminfo;（我們一直關(guān)注的內(nèi)存信息哦～）提供信息。具體的獲取信息的函數(shù)（同樣位于setup.c (archarmkernel)）：
  1. int __init arm_add_memory(phys_addr_t start, unsigned long size)
  2. {
  3. struct membank *bank = &meminfo.bank[meminfo.nr_banks];
  5. if (meminfo.nr_banks >= NR_BANKS) {
  6. printk(KERN_CRIT "NR_BANKS too low, "
  7. "ignoring memory at 0xllxn", (long long)start);
  8. return -EINVAL;
  9. }
  11. /*
  12. * Ensure that start/size are aligned to a page boundary.
  13. * Size is appropriately rounded down, start is rounded up.
  14. */
  15. size -= start & ~PAGE_MASK;
  16. bank->start = PAGE_ALIGN(start);
  17. bank->size = size & PAGE_MASK;
  19. /*
  20. * Check whether this memory region has non-zero size or
  21. * invalid node number.
  22. */
  23. if (bank->size == 0)
  24. return -EINVAL;
  26. meminfo.nr_banks++;
  27. return 0;
  28. }
  30. /*
  31. * Pick out the memory size. We look for mem=size@start,
  32. * where start and size are "size[KkMm]"
  33. */
  34. static int __init early_mem(char *p)
  35. {
  36. static int usermem __initdata = 0;
  37. unsigned long size;
  38. phys_addr_t start;
  39. char *endp;
  41. /*
  42. * If the user specifies memory size, we
  43. * blow away any automatically generated
  44. * size.
  45. */
  46. if (usermem == 0) {
  47. usermem = 1;
  48. meminfo.nr_banks = 0;
  49. }
  51. start = PHYS_OFFSET;
  52. size = memparse(p, &endp);
  53. if (*endp == @)
  54. start = memparse(endp + 1, NULL);
  56. arm_add_memory(start, size);
  58. return 0;
  59. }
  60. early_param("mem", early_mem);
vmalloc=size參數(shù)，她為初始化vmalloc_min（需要保留的內(nèi)核虛擬地址空間大小，也就是這個(gè)內(nèi)核虛擬地址空間中除去邏輯地址空間和必要的防止越界的保護(hù)空洞后最少要預(yù)留的地址空間）提供信息。具體的實(shí)現(xiàn)函數(shù)（位于mmu.c (archarmmm)）：
  1. static void * __initdata vmalloc_min = (void *)(VMALLOC_END - SZ_128M);
  2. /*
  3. * vmalloc=size forces the vmalloc area to be exactly size
  4. * bytes. This can be used to increase (or decrease) the vmalloc
  5. * area - the default is 128m.
  6. */
  7. static int __init early_vmalloc(char *arg)
  8. {
  9. unsigned long vmalloc_reserve = memparse(arg, NULL);
  11. if (vmalloc_reserve < SZ_16M) {
  12. vmalloc_reserve = SZ_16M;
  13. printk(KERN_WARNING
  14. "vmalloc area too small, limiting to %luMBn",
  15. vmalloc_reserve >> 20);
  16. }
  18. if (vmalloc_reserve > VMALLOC_END - (PAGE_OFFSET + SZ_32M)) {
  19. vmalloc_reserve = VMALLOC_END - (PAGE_OFFSET + SZ_32M);
  20. printk(KERN_WARNING
  21. "vmalloc area is too big, limiting to %luMBn",
  22. vmalloc_reserve >> 20);
  23. }
  25. vmalloc_min = (void *)(VMALLOC_END - vmalloc_reserve);
  27. return 0;
  28. }
  29. early_param("vmalloc", early_vmalloc);
（2）在獲得了必要的信息（初始化好struct meminfo meminfo和vmalloc_min）后，內(nèi)核通過sanity_check_meminfo函數(shù)自動(dòng)去通過vmalloc_min信息來初始化每個(gè)meminfo.bank[？]中的highmem成員。此過程中如果有必要，將可能會(huì)改變meminfo中的bank數(shù)組。處理函數(shù)位于mmu.c (archarmmm)：
  1. static phys_addr_t lowmem_limit __initdata = 0;
  3. void __init sanity_check_meminfo(void)
  4. {
  5. int i, j, highmem = 0;
  7. for (i = 0, j = 0; i < meminfo.nr_banks; i++) {
  8. struct membank *bank = &meminfo.bank[j];
  9. *bank = meminfo.bank[i];
  11. #ifdef CONFIG_HIGHMEM
  12. if (__va(bank->start) >= vmalloc_min ||
  13. __va(bank->start) < (void *)PAGE_OFFSET)
  14. highmem = 1;
  16. bank->highmem = highmem;
  18. /*
  19. * Split those memory banks which are partially overlapping
  20. * the vmalloc area greatly simplifying things later.
  21. */
  22. if (__va(bank->start) < vmalloc_min &&
  23. bank->size > vmalloc_min - __va(bank->start)) {
  24. if (meminfo.nr_banks >= NR_BANKS) {
  25. printk(KERN_CRIT "NR_BANKS too low, "
  26. "ignoring high memoryn");
  27. } else {
  28. memmove(bank + 1, bank,
  29. (meminfo.nr_banks - i) * sizeof(*bank));
  30. meminfo.nr_banks++;
  31. i++;
  32. bank[1].size -= vmalloc_min - __va(bank->start);
  33. bank[1].start = __pa(vmalloc_min - 1) + 1;
  34. bank[1].highmem = highmem = 1;
  35. j++;
  36. }
  37. bank->size = vmalloc_min - __va(bank->start);
  38. }
  39. #else
  40. bank->highmem = highmem;
  42. /*
  43. * Check whether this memory bank would entirely overlap
  44. * the vmalloc area.
  45. */
  46. if (__va(bank->start) >= vmalloc_min ||
  47. __va(bank->start) < (void *)PAGE_OFFSET) {
  48. printk(KERN_NOTICE "Ignoring RAM at %.8llx-%.8llx "
  49. "(vmalloc region overlap).n",
  50. (unsigned long long)bank->start,
  51. (unsigned long long)bank->start + bank->size - 1);
  52. continue;
  53. }
  55. /*
  56. * Check whether this memory bank would partially overlap
  57. * the vmalloc area.
  58. */
  59. if (__va(bank->start + bank->size) > vmalloc_min ||
  60. __va(bank->start + bank->size) < __va(bank->start)) {
  61. unsigned long newsize = vmalloc_min - __va(bank->start);
  62. printk(KERN_NOTICE "Truncating RAM at %.8llx-%.8llx "
  63. "to -%.8llx (vmalloc region overlap).n",
  64. (unsigned long long)bank->start,
  65. (unsigned long long)bank->start + bank->size - 1,
  66. (unsigned long long)bank->start + newsize - 1);
  67. bank->size = newsize;
  68. }
  69. #endif
  70. if (!bank->highmem && bank->start + bank->size > lowmem_limit)
  71. lowmem_limit = bank->start + bank->size;
  73. j++;
  74. }
  75. #ifdef CONFIG_HIGHMEM
  76. if (highmem) {
  77. const char *reason = NULL;
  79. if (cache_is_vipt_aliasing()) {
  80. /*
  81. * Interactions between kmap and other mappings
  82. * make highmem support with aliasing VIPT caches
  83. * rather difficult.
  84. */
  85. reason = "with VIPT aliasing cache";
  86. }
  87. if (reason) {
  88. printk(KERN_CRIT "HIGHMEM is not supported %s, ignoring high memoryn",
  89. reason);
  90. while (j > 0 && meminfo.bank[j - 1].highmem)
  91. j--;
  92. }
  93. }
  94. #endif
  95. meminfo.nr_banks = j;
  96. memblock_set_current_limit(lowmem_limit);
  97. }
（3）最后必須做的就是排序了，完成了這個(gè)工作就可以完全被我們上面提到的find_limits函數(shù)使用了，而這個(gè)工作就放在了接下來的arm_memblock_init(&meminfo, mdesc);中的一開頭：
  1. static int __init meminfo_cmp(const void *_a, const void *_b)
  2. {
  3. const struct membank *a = _a, *b = _b;
  4. long cmp = bank_pfn_start(a) - bank_pfn_start(b);
  5. return cmp < 0 ? -1 : cmp > 0 ? 1 : 0;
  6. }
  8. void __init arm_memblock_init(struct meminfo *mi, struct machine_desc *mdesc)
  9. {
  10. int i;
  12. sort(&meminfo.bank, meminfo.nr_banks, sizeof(meminfo.bank[0]), meminfo_cmp, NULL);
  14. memblock_init();
  15. for (i = 0; i < mi->nr_banks; i++)
  16. memblock_add(mi->bank[i].start, mi->bank[i].size);
  18. /* Register the kernel text, kernel data and initrd with memblock. */
  19. #ifdef CONFIG_XIP_KERNEL
  20. memblock_reserve(__pa(_sdata), _end - _sdata);
  21. #else
  22. memblock_reserve(__pa(_stext), _end - _stext);
  23. #endif
  24. #ifdef CONFIG_BLK_DEV_INITRD
  25. if (phys_initrd_size &&
  26. !memblock_is_region_memory(phys_initrd_start, phys_initrd_size)) {
  27. pr_err("INITRD: 0xlx+0xlx is not a memory region - disabling initrdn",
  28. phys_initrd_start, phys_initrd_size);
  29. phys_initrd_start = phys_initrd_size = 0;
  30. }
  31. if (phys_initrd_size &&
  32. memblock_is_region_reserved(phys_initrd_start, phys_initrd_size)) {
  33. pr_err("INITRD: 0xlx+0xlx overlaps in-use memory region - disabling initrdn",
  34. phys_initrd_start, phys_initrd_size);
  35. phys_initrd_start = phys_initrd_size = 0;
  36. }
  37. if (phys_initrd_size) {
  38. memblock_reserve(phys_initrd_start, phys_initrd_size);
  40. /* Now convert initrd to virtual addresses */
  41. initrd_start = __phys_to_virt(phys_initrd_start);
  42. initrd_end = initrd_start + phys_initrd_size;
  43. }
  44. #endif
  46. arm_mm_memblock_reserve();
  47. arm_dt_memblock_reserve();
  49. /* reserve any platform specific memblock areas */
  50. if (mdesc->reserve)
  51. mdesc->reserve();
  53. memblock_analyze();
  54. memblock_dump_all();
  55. }
通過上面的分析，整個(gè)高低端內(nèi)存是如何確定的基本就清晰了，這里總結(jié)一下：
ARM構(gòu)架中，高-低段內(nèi)存是內(nèi)核通過內(nèi)核啟動(dòng)參數(shù)（mem=size@start和vmalloc=size）來自動(dòng)配置的，如果沒有特殊去配置他，那么在普通的ARM系統(tǒng)中是不會(huì)有高端內(nèi)存存在的。除非你系統(tǒng)的RAM很大或vmalloc配置得很大，就很可能出現(xiàn)高端內(nèi)存。

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