3.4.2 特定于体系结构的设置 （二）：内存初始化步骤
初始化步骤
在内核已经载入内存、而初始化的汇编程序部分已经执行完毕后，内核必须执行哪些特定于系统的步骤呢?图3-12给出了各个操作的代码流程图。
start_kernel
->setup_arch
->machine_specific_memory_setup
->setup_memory
->paging_init
-> free_area_init
->free_area_init_node
（1）首先调用machine_specific_memory_setup（3.18.3版本直接在setup_arch中完成，而取消了machine_specific_memory_setup，该函数实现可以参考2.6.24）创建链表，包括系统占据内存区和空闲内存区。由于IA-32家族的各个子体系结构获得该信息 方式稍有不同，内核提供了一个特定于机器的函数，定义在include/asm-x86/mach-type/setup.c中，type可以是default、voyager或visws。这里只讨论default的情况。
BIOS提供的映射给出了在这种情况下使用的各个内存区，在系统启动时，找到的内存区由内核函数print_memory_map显示。
static void __init print_memory_map(void)
{
int i;
const int field = 2 * sizeof(unsigned long);
for (i = 0; i < boot_mem_map.nr_map; i++) {
printk(KERN_INFO " memory: %0*Lx @ %0*Lx ",
field, (unsigned long long) boot_mem_map.map[i].size,
field, (unsigned long long) boot_mem_map.map[i].addr);
switch (boot_mem_map.map[i].type) {
case BOOT_MEM_RAM:
printk(KERN_CONT "(usable)\n");
break;
case BOOT_MEM_INIT_RAM:
printk(KERN_CONT "(usable after init)\n");
break;
case BOOT_MEM_ROM_DATA:
printk(KERN_CONT "(ROM data)\n");
break;
case BOOT_MEM_RESERVED:
printk(KERN_CONT "(reserved)\n");
break;
default:
printk(KERN_CONT "type %lu\n", boot_mem_map.map[i].type);
break;
}
}
}
打印出来的信息类似于（取自x86 baytrail系统）：
[ 0.000000] e820: BIOS-provided physical RAM map:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009c7ff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009c800-0x000000000009ffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000001fffffff] usable
[ 0.000000] BIOS-e820: [mem 0x0000000020000000-0x00000000200fffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000020100000-0x000000006d590fff] usable
[ 0.000000] BIOS-e820: [mem 0x000000006d591000-0x000000006d5c0fff] reserved
[ 0.000000] BIOS-e820: [mem 0x000000006d5c1000-0x000000006d5d0fff] ACPI data
[ 0.000000] BIOS-e820: [mem 0x000000006d5d1000-0x000000006d715fff] ACPI NVS
[ 0.000000] BIOS-e820: [mem 0x000000006d716000-0x000000006da79fff] reserved
[ 0.000000] BIOS-e820: [mem 0x000000006da7a000-0x000000006da7afff] usable
[ 0.000000] BIOS-e820: [mem 0x000000006da7b000-0x000000006dabcfff] reserved
[ 0.000000] BIOS-e820: [mem 0x000000006dabd000-0x000000006dc2afff] usable
[ 0.000000] BIOS-e820: [mem 0x000000006dc2b000-0x000000006dff9fff] reserved
[ 0.000000] BIOS-e820: [mem 0x000000006dffa000-0x000000006dffffff] usable
[ 0.000000] BIOS-e820: [mem 0x00000000e00f8000-0x00000000e00f8fff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fed01000-0x00000000fed01fff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000ffb00000-0x00000000ffffffff] reserved
如果BIOS没有提供信息，内核自身生成一个表，将0-640KB和1MB之前的内存标记为可用。
（2）内核接下来调用parse_cmd_early（这个在3.18.3中仍被取代，直接在setup_arch中实现，可参考2.6.24）分析命令行，主要关注类似mem=xxx、highmem=xxx或memmap=xxx之类的参数。如果内核计算的值或BIOS提供的值不正确，管理员可以修改可用内存的数量或手动划定内存区。该选项只适用于比较古老的计算机。Highmem-允许修改检测到的高端内存域的长度。它可用于内存配置非常大的计算机，以限定可用的内存的数量，因为超大内存有时会导致性能下降。
（3）下一个主要步骤在setup_memory中执行。其主要功能
A. 确定可用的物理内存的数目
B. 初始化bootmem分配器
C. 接下来分配各种内存区，如运行第一个用户空间过程所需的最初的RAM磁盘。
setup_memory(void *kernel_end)
{
struct memclust_struct * cluster;
struct memdesc_struct * memdesc;
unsigned long start_kernel_pfn, end_kernel_pfn;
unsigned long bootmap_size, bootmap_pages, bootmap_start;
unsigned long start, end;
unsigned long i;
/* Find free clusters, and init and free the bootmem accordingly. */
memdesc = (struct memdesc_struct *)
(hwrpb->mddt_offset + (unsigned long) hwrpb);
for_each_mem_cluster(memdesc, cluster, i) {
printk("memcluster %lu, usage %01lx, start %8lu, end %8lu\n",
i, cluster->usage, cluster->start_pfn,
cluster->start_pfn + cluster->numpages);
/* Bit 0 is console/PALcode reserved. Bit 1 is
non-volatile memory -- we might want to mark
this for later. */
if (cluster->usage & 3)
continue;
end = cluster->start_pfn + cluster->numpages;
if (end > max_low_pfn)
max_low_pfn = end;
}
/*
* Except for the NUMA systems (wildfire, marvel) all of the
* Alpha systems we run on support 32GB of memory or less.
* Since the NUMA systems introduce large holes in memory addressing,
* we can get into a situation where there is not enough contiguous
* memory for the memory map.
*
* Limit memory to the first 32GB to limit the NUMA systems to
* memory on their first node (wildfire) or 2 (marvel) to avoid
* not being able to produce the memory map. In order to access
* all of the memory on the NUMA systems, build with discontiguous
* memory support.
*
* If the user specified a memory limit, let that memory limit stand.
*/
if (!mem_size_limit)
mem_size_limit = (32ul * 1024 * 1024 * 1024) >> PAGE_SHIFT;
if (mem_size_limit && max_low_pfn >= mem_size_limit)
{
printk("setup: forcing memory size to %ldK (from %ldK).\n",
mem_size_limit << (PAGE_SHIFT - 10),
max_low_pfn << (PAGE_SHIFT - 10));
max_low_pfn = mem_size_limit;
}
/* Find the bounds of kernel memory. */
start_kernel_pfn = PFN_DOWN(KERNEL_START_PHYS);
end_kernel_pfn = PFN_UP(virt_to_phys(kernel_end));
bootmap_start = -1;
try_again:
if (max_low_pfn <= end_kernel_pfn)
panic("not enough memory to boot");
/* We need to know how many physically contiguous pages
we'll need for the bootmap. */
bootmap_pages = bootmem_bootmap_pages(max_low_pfn);
/* Now find a good region where to allocate the bootmap. */
for_each_mem_cluster(memdesc, cluster, i) {
if (cluster->usage & 3)
continue;
start = cluster->start_pfn;
end = start + cluster->numpages;
if (start >= max_low_pfn)
continue;
if (end > max_low_pfn)
end = max_low_pfn;
if (start < start_kernel_pfn) {
if (end > end_kernel_pfn
&& end - end_kernel_pfn >= bootmap_pages) {
bootmap_start = end_kernel_pfn;
break;
} else if (end > start_kernel_pfn)
end = start_kernel_pfn;
} else if (start < end_kernel_pfn)
start = end_kernel_pfn;
if (end - start >= bootmap_pages) {
bootmap_start = start;
break;
}
}
if (bootmap_start == ~0UL) {
max_low_pfn >>= 1;
goto try_again;
}
/* Allocate the bootmap and mark the whole MM as reserved. */
bootmap_size = init_bootmem(bootmap_start, max_low_pfn);
/* Mark the free regions. */
for_each_mem_cluster(memdesc, cluster, i) {
if (cluster->usage & 3)
continue;
start = cluster->start_pfn;
end = cluster->start_pfn + cluster->numpages;
if (start >= max_low_pfn)
continue;
if (end > max_low_pfn)
end = max_low_pfn;
if (start < start_kernel_pfn) {
if (end > end_kernel_pfn) {
free_bootmem(PFN_PHYS(start),
(PFN_PHYS(start_kernel_pfn)
- PFN_PHYS(start)));
printk("freeing pages %ld:%ld\n",
start, start_kernel_pfn);
start = end_kernel_pfn;
} else if (end > start_kernel_pfn)
end = start_kernel_pfn;
} else if (start < end_kernel_pfn)
start = end_kernel_pfn;
if (start >= end)
continue;
free_bootmem(PFN_PHYS(start), PFN_PHYS(end) - PFN_PHYS(start));
printk("freeing pages %ld:%ld\n", start, end);
}
/* Reserve the bootmap memory. */
reserve_bootmem(PFN_PHYS(bootmap_start), bootmap_size,
BOOTMEM_DEFAULT);
printk("reserving pages %ld:%ld\n", bootmap_start, bootmap_start+PFN_UP(bootmap_size));
#ifdef CONFIG_BLK_DEV_INITRD
initrd_start = INITRD_START;
if (initrd_start) {
initrd_end = initrd_start+INITRD_SIZE;
printk("Initial ramdisk at: 0x%p (%lu bytes)\n",
(void *) initrd_start, INITRD_SIZE);
if ((void *)initrd_end > phys_to_virt(PFN_PHYS(max_low_pfn))) {
if (!move_initrd(PFN_PHYS(max_low_pfn)))
printk("initrd extends beyond end of memory "
"(0x%08lx > 0x%p)\ndisabling initrd\n",
initrd_end,
phys_to_virt(PFN_PHYS(max_low_pfn)));
} else {
reserve_bootmem(virt_to_phys((void *)initrd_start),
INITRD_SIZE, BOOTMEM_DEFAULT);
}
}
#endif /* CONFIG_BLK_DEV_INITRD */
}
（4）paging_init初始化内核页表并启动内存分页，因为IA-32计算机上默认情况下分页是禁用的，如果内核编译了PAE，而且处理器也支持Execute Disable Protection，则启用该特性。通过调用pagetable_init，该函数确保了直接映射到内核地址空间的物理内存被初始化。低端内存中的所有页帧都直接映射到PAGE_OFFSET之上的虚拟内存区。这使得内核无需处理页表，即可寻址相当一部分可用内存。
void __init paging_init(void)
{
unsigned long zones_size[MAX_NR_ZONES] = {0, };
unsigned long dma_pfn, high_pfn;
dma_pfn = virt_to_phys((char *)MAX_DMA_ADDRESS) >> PAGE_SHIFT;
high_pfn = max_pfn = max_low_pfn;
if (dma_pfn >= high_pfn)
zones_size[ZONE_DMA] = high_pfn;
else {
zones_size[ZONE_DMA] = dma_pfn;
zones_size[ZONE_NORMAL] = high_pfn - dma_pfn;
}
/* Initialize mem_map[]. */
free_area_init(zones_size);
/* Initialize the kernel's ZERO_PGE. */
memset((void *)ZERO_PGE, 0, PAGE_SIZE);
}
（5）Zone_sizes_init（3.18.3中不存在，已合并到paging_init中），初始化系统中所有结点pgdat_t的实例。首先使用add_active_range，对可用的物理内存建立一个相对简单的列表。体系结构无关的函数free_area_init_nodes接下来使用该信息建立完备的内核数据结构。这是一个非常重要的步骤，对内核在运行时管理页帧的方式.
void __init free_area_init(unsigned long *zones_size)
{
free_area_init_node(0, zones_size,
__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
}
void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
unsigned long node_start_pfn, unsigned long *zholes_size)
{
pg_data_t *pgdat = NODE_DATA(nid);
unsigned long start_pfn = 0;
unsigned long end_pfn = 0;
/* pg_data_t should be reset to zero when it's allocated */
WARN_ON(pgdat->nr_zones || pgdat->classzone_idx);
pgdat->node_id = nid;
pgdat->node_start_pfn = node_start_pfn;
#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
printk(KERN_INFO "Initmem setup node %d [mem %#010Lx-%#010Lx]\n", nid,
(u64) start_pfn << PAGE_SHIFT, (u64) (end_pfn << PAGE_SHIFT) - 1);
#endif
calculate_node_totalpages(pgdat, start_pfn, end_pfn,
zones_size, zholes_size);
alloc_node_mem_map(pgdat);
#ifdef CONFIG_FLAT_NODE_MEM_MAP
printk(KERN_DEBUG "free_area_init_node: node %d, pgdat %08lx, node_mem_map %08lx\n",
nid, (unsigned long)pgdat,
(unsigned long)pgdat->node_mem_map);
#endif
free_area_init_core(pgdat, start_pfn, end_pfn,
zones_size, zholes_size);
}