在内核建立相应的内存管理区之前,先要进行物理内存的探测,获取相关的内存信息。对于X86架构,内核在void main()中调用detect_memory()来进行物理内存的探测。
- void main(void)
- {
- ...
- ...
- /* Detect memory layout */
- detect_memory();
- ...
- ...
- }
detect_memory()中分别通过调用0xE820H,0xE801H和0x88H从BIOS中获取内存布局和相关的信息。
- <span style="font-size:12px;">int detect_memory(void)
- {
- int err = -1;
- if (detect_memory_e820() > 0)
- </span><span style="font-size:12px;"> err = 0;
- if (!detect_memory_e801())
- err = 0;
- if (!detect_memory_88())
- err = 0;
- return err;
- }
- </span>
这三个函数都通过int 0x15触发BIOS中断来获取相关信息
下面我们着重分析detect_memory_e820().在此之前先了解一个关键的数据结构--struct e820entry.该结构用来保存一个物理内存段的地址信息以及类型。
- <span style="font-size:12px;">struct e820entry {
- __u64 addr; /* start of memory segment */
- __u64 size; /* size of memory segment */
- __u32 type; /* type of memory segment */
- } __attribute__((packed));</span>
addr:该内存段的起始地址
size:该内存段的大小
type:该内存段的类型,可分为Usable (normal) RAM,Reserved - unusable,ACPI reclaimable memory,ACPI NVS memory,Area containing bad memory,要获取所有的内存段的信息,detect_memory_e820()通过一个do_while循环来不断触发int 0x15中断来获取每个内存段的信息,并且将这些信息保存在一个struct e820entry类型的数组中,即boot_params.e820_map,下面是相关代码
- <span style="font-size:12px;">static int detect_memory_e820(void)
- {
- int count = 0;
- struct biosregs ireg, oreg;
- struct e820entry *desc = boot_params.e820_map;
- static struct e820entry buf; /* static so it is zeroed */
- initregs(&ireg);
- ireg.ax = 0xe820;
- ireg.cx = sizeof buf;
- ireg.edx = SMAP;
- ireg.di = (size_t)&buf;
- /*
- * Note: at least one BIOS is known which assumes that the
- * buffer pointed to by one e820 call is the same one as
- * the previous call, and only changes modified fields. Therefore,
- * we use a temporary buffer and copy the results entry by entry.
- *
- * This routine deliberately does not try to account for
- * ACPI 3+ extended attributes. This is because there are
- * BIOSes in the field which report zero for the valid bit for
- * all ranges, and we don't currently make any use of the
- * other attribute bits. Revisit this if we see the extended
- * attribute bits deployed in a meaningful way in the future.
- */
- do {
- intcall(0x15, &ireg, &oreg); /*触发0x15中断*/
- ireg.ebx = oreg.ebx; /* for next iteration... */
- /* BIOSes which terminate the chain with CF = 1 as opposed
- to %ebx = 0 don't always report the SMAP signature on
- the final, failing, probe. */
- if (oreg.eflags & X86_EFLAGS_CF)
- break;
- /* Some BIOSes stop returning SMAP in the middle of
- the search loop. We don't know exactly how the BIOS
- screwed up the map at that point, we might have a
- partial map, the full map, or complete garbage, so
- just return failure. */
- if (oreg.eax != SMAP) {
- count = 0;
- break;
- }
- *desc++ = buf;/*保存获取的内存段信息*/
- count++; /*获取的内存段数目加1*/
- } while (ireg.ebx && count < ARRAY_SIZE(boot_params.e820_map));
- return boot_params.e820_entries = count;
- }</span>
其他两个函数还没弄太明白,只知道是用来获取扩展内存的大小的~其中0x88H获取的内存上限为64M(以1K为单位),而0xe801可以获取到64M以上的内存(以64K为单位),它们也将获取的信息保存在boot_params中以供后用。
在start_kernel()-->setup_arch()中,我们可以看到很多与架构相关的初始化工作,接下来我们选择几个与内存管理相关的关键函数进行分析。
- <span style="font-size:12px;">void __init setup_arch(char **cmdline_p)
- {
- ...
- ...
- setup_memory_map();
- ...
- ...
- max_pfn = e820_end_of_ram_pfn(); /*找到最大的页框编号*/
- ...
- /* max_low_pfn get updated here */
- find_low_pfn_range();/*设定高、低内存的分界线*/
- ...
- ...
- }
- </span>
首先来看setup_memory_map().
- void __init setup_memory_map(void)
- {
- char *who;
- who = x86_init.resources.memory_setup();
- memcpy(&e820_saved, &e820, sizeof(struct e820map));
- printk(KERN_INFO "BIOS-provided physical RAM map:\n");
- e820_print_map(who);
- }
该函数中,通过结构体x86_init_resources调用memory_setup(),在x86_init.c中,我们可以看到该函数指针的初始化
- <span style="font-size:12px;">struct x86_init_ops x86_init __initdata = {
- .resources = {
- .probe_roms = x86_init_noop,
- .reserve_resources = reserve_standard_io_resources,
- .memory_setup = default_machine_specific_memory_setup,
- },</span>
跟踪找到default_machine_specific_memory_setup(),
- <span style="font-size:12px;"></span>
- <span style="font-size:12px;">char *__init default_machine_specific_memory_setup(void)
- {
- char *who = "BIOS-e820";
- u32 new_nr;
- /*
- * Try to copy the BIOS-supplied E820-map.
- *
- * Otherwise fake a memory map; one section from 0k->640k,
- * the next section from 1mb->appropriate_mem_k
- */
- new_nr = boot_params.e820_entries;
- sanitize_e820_map(boot_params.e820_map, /*消除重叠的内存段*/
- ARRAY_SIZE(boot_params.e820_map),
- &new_nr);
- boot_params.e820_entries = new_nr;
- /*将内存布局的信息从boot_params.e820_map拷贝到struct e820map e820*/
- if (append_e820_map(boot_params.e820_map, boot_params.e820_entries)
- < 0) {
- u64 mem_size;
- /* compare results from other methods and take the greater */
- if (boot_params.alt_mem_k
- < boot_params.screen_info.ext_mem_k) {
- mem_size = boot_params.screen_info.ext_mem_k;
- who = "BIOS-88";
- } else {
- mem_size = boot_params.alt_mem_k;
- who = "BIOS-e801";
- }
- e820.nr_map = 0;
- e820_add_region(0, LOWMEMSIZE(), E820_RAM);
- e820_add_region(HIGH_MEMORY, mem_size << 10, E820_RAM);
- }
- /* In case someone cares... */
- return who;
- }</span>
可以看到该函数主要完成两个功能:
1.消除内存段的重叠部分
2.将内存布局信息从boot_params.e820_map拷贝到e820中
跟踪append_e820_map()-->__append_e820_map()
- <span style="font-size:12px;">static int __init __append_e820_map(struct e820entry *biosmap, int nr_map)
- {
- while (nr_map) {
- u64 start = biosmap->addr;
- u64 size = biosmap->size;
- u64 end = start + size;
- u32 type = biosmap->type;
- /* Overflow in 64 bits? Ignore the memory map. */
- if (start > end)
- return -1;
- e820_add_region(start, size, type);
- biosmap++;
- nr_map--;
- }
- return 0;
- }</span>
其中传给biosmap的参数为boot_params.e820_map,即内存布局的起始地址,传给nr_map的参数为boot_params.e820_entries,即获取的内存段的数目。在该函数中,对每个段调用e820_add_region,来看看这个函数
- <span style="font-size:12px;">void __init e820_add_region(u64 start, u64 size, int type)
- {
- __e820_add_region(&e820, start, size, type);
- }</span>
- <span style="font-size:12px;">static void __init __e820_add_region(struct e820map *e820x, u64 start, u64 size,
- int type)
- {
- int x = e820x->nr_map;
- if (x >= ARRAY_SIZE(e820x->map)) {
- printk(KERN_ERR "Ooops! Too many entries in the memory map!\n");
- return;
- }
- e820x->map[x].addr = start;
- e820x->map[x].size = size;
- e820x->map[x].type = type;
- e820x->nr_map++;
- }</span>
最终由__e820_add_region()函数将内存段的信息保存到e820的map数组中。
下面再来看看e820_end_of_ram_pfn(),该函数用来遍历所有内存段的页框,找到低端内存的最大页框编号
- <span style="font-size:12px;">unsigned long __init e820_end_of_ram_pfn(void)
- {
- return e820_end_pfn(MAX_ARCH_PFN, E820_RAM);
- }</span>
E820_RAM代表可用内存
MAX_ARCH_PFN的定义如下
- <span style="font-size:12px;">#ifdef CONFIG_X86_32
- # ifdef CONFIG_X86_PAE
- # define MAX_ARCH_PFN (1ULL<<(36-PAGE_SHIFT))
- # else
- # define MAX_ARCH_PFN (1ULL<<(32-PAGE_SHIFT))
- # endif
- #else /* CONFIG_X86_32 */
- # define MAX_ARCH_PFN MAXMEM>>PAGE_SHIFT
- #endif</span>
这里可以看到如果定义了CONFIG_X86_32,那么在没使用PAE的情况下,MAX_ARCH_PFN的值就为4GB内存对应的页框号,而如果定义了则MAX_ARCH_PFN的值为MAXMEM对应的页框号,我们来看MAXMEM的定义:
- <span style="font-size:12px;">#define MAXMEM (VMALLOC_END - PAGE_OFFSET - __VMALLOC_RESERVE)
- </span>
这里面的宏都和线性地址的最后1GB有关,其中__VANALLOC_RESERVE为128M,下图说明了第4GB的内存划分
结合这个图我们可以得出MAXMEM为一个略小于896M的值(896M-8K-4M-4M),即略小于低端内存的上限,高端内存的起始地址,这样是为了避免某些情况下产生的OOPS。
走得貌似有点远了,再回到e820_end_of_ram_pfn(),跟踪e820_end_pfn(MAX_ARCH_PFN, E820_RAM):
- <span style="font-size:12px;">static unsigned long __init e820_end_pfn(unsigned long limit_pfn, unsigned type)
- {
- int i;
- unsigned long last_pfn = 0;
- unsigned long max_arch_pfn = MAX_ARCH_PFN;
- for (i = 0; i < e820.nr_map; i++) { /*循环遍历内存布局数组*/
- struct e820entry *ei = &e820.map[i];
- unsigned long start_pfn;
- unsigned long end_pfn;
- if (ei->type != type)
- continue;
- start_pfn = ei->addr >> PAGE_SHIFT;
- end_pfn = (ei->addr + ei->size) >> PAGE_SHIFT;
- if (start_pfn >= limit_pfn)/*起始地址大于MAX_ARCH_PFN,无视之*/
- continue;
- if (end_pfn > limit_pfn) { /*结束地址大于MAX_ARCH_PFN则直接最大页框编号设为MAX_ARCH_PFN*/
- last_pfn = limit_pfn;
- break;
- }
- if (end_pfn > last_pfn) /*该内存段的末地址大于之前找到的最大页框编号,
- 则重置最大页框编号*/
- last_pfn = end_pfn;
- }
- if (last_pfn > max_arch_pfn)
- last_pfn = max_arch_pfn;
- printk(KERN_INFO "last_pfn = %#lx max_arch_pfn = %#lx\n",
- last_pfn, max_arch_pfn);
- return last_pfn;
- }</span>
最后分析一个函数,setup_arch()-->find_low_pfn_range().该函数用来划分低端内存和高端内存的界限,也可以说是确定高端内存的起始地址。再看这个函数之前,还得先看几个宏定义
- <span style="font-size:12px;">#define MAXMEM_PFN PFN_DOWN(MAXMEM)
- #define PFN_UP(x) (((x) + PAGE_SIZE-1) >> PAGE_SHIFT)
- #define PFN_DOWN(x) ((x) >> PAGE_SHIFT)</span>
可以看出
PFN_DOWN(x)计算出地址值x对应的页框的编号
PFN_UP(x)计算出地址值对应的页框的下一个页框的编号
MAXMEM_PFN为MAXMEM对应的页框编号
再看具体的代码:
- <span style="font-size:12px;">void __init find_low_pfn_range(void)
- {
- /* it could update max_pfn */
- if (max_pfn <= MAXMEM_PFN) /*实际物理内存小于等于低端内存896M*/
- lowmem_pfn_init();
- else /*实际的物理内存大于896M*/
- highmem_pfn_init();
- }</span>
当实际的物理内存小于等于低端内存时,由lowmem_pfn_init()进行分配
- <span style="font-size:12px;">void __init lowmem_pfn_init(void)
- {
- /* max_low_pfn is 0, we already have early_res support */
- /*将分界线初始化为实际物理内存的最大页框号,由于系统的内存小于896M,
- 所以全部内存为低端内存,如需要高端内存,则从中分一部分出来进行分配*/
- max_low_pfn = max_pfn;
- if (highmem_pages == -1)
- highmem_pages = 0;
- #ifdef CONFIG_HIGHMEM /*如果用户定义了HIGHMEM,即需要分配高端内存*/
- if (highmem_pages >= max_pfn) { /*如果高端内存的页起始地址>=最大页框号,则无法分配*/
- printk(KERN_ERR MSG_HIGHMEM_TOO_BIG,
- pages_to_mb(highmem_pages), pages_to_mb(max_pfn));
- highmem_pages = 0;
- }
- if (highmem_pages) {
- /*这个条件保证低端内存不能小于64M*/
- if (max_low_pfn - highmem_pages < 64*1024*1024/PAGE_SIZE) {
- printk(KERN_ERR MSG_LOWMEM_TOO_SMALL,
- pages_to_mb(highmem_pages));
- highmem_pages = 0;
- }
- max_low_pfn -= highmem_pages; /*设定好低、高端内存的分界线*/
- }
- #else
- if (highmem_pages)
- printk(KERN_ERR "ignoring highmem size on non-highmem kernel!\n");
- #endif
- }
- </span>
当实际的物理内存大于896M,由highmem_pfn_init()进行分配
- <span style="font-size:12px;">void __init highmem_pfn_init(void)
- {
- max_low_pfn = MAXMEM_PFN; /*设定高端内存和低端内存的分界线*/
- if (highmem_pages == -1) /*未设定高端内存的页框数*/
- highmem_pages = max_pfn - MAXMEM_PFN; /*默认为最大页框数减去MAXMEM_PFN*/
- if (highmem_pages + MAXMEM_PFN < max_pfn) /*高端内存页框数加上MAXMEM_PFN小于最大页框数*/
- max_pfn = MAXMEM_PFN + highmem_pages; /*将最大页框数下调到前两者的和*/
- if (highmem_pages + MAXMEM_PFN > max_pfn){ /*申请的高端内存超过范围则不分配*/
- printk(KERN_WARNING MSG_HIGHMEM_TOO_SMALL,
- pages_to_mb(max_pfn - MAXMEM_PFN),
- pages_to_mb(highmem_pages));
- highmem_pages = 0;
- }
- #ifndef CONFIG_HIGHMEM
- /* Maximum memory usable is what is directly addressable */
- printk(KERN_WARNING "Warning only %ldMB will be used.\n", MAXMEM>>20);
- if (max_pfn > MAX_NONPAE_PFN)
- printk(KERN_WARNING "Use a HIGHMEM64G enabled kernel.\n");
- else
- printk(KERN_WARNING "Use a HIGHMEM enabled kernel.\n");
- max_pfn = MAXMEM_PFN;
- #else /* !CONFIG_HIGHMEM */
- #ifndef CONFIG_HIGHMEM64G
- if (max_pfn > MAX_NONPAE_PFN) {
- max_pfn = MAX_NONPAE_PFN;
- printk(KERN_WARNING MSG_HIGHMEM_TRIMMED);
- }
- #endif /* !CONFIG_HIGHMEM64G */
- #endif /* !CONFIG_HIGHMEM */
- }
- </span>
至此,已将内存探测和高低端内存的划分分析完了,接下来再分析管理区的初始化。