我们在阅读源码时，函数功能可以分为两类：1. bootmem.c 2. page_alloc.c。
1. bootmem.c是关于bootmem allocator的，上篇文章已经简述过。
2. page_alloc.c是关于Memory Management subsystem的。
关于内存管理子系统的初始化调用了多个函数，我们首先分析在bootmem_init中调用的add_active_range函数。
start_kernel –> setup_arch —> arch_mem_init —> bootmem_init —> add_active_range.
通过上述分析，我们此时可以确认内存信息，那么我们确认的内存信息必然要告诉Memory Management subsystem，首先通过add_active_range完成。
调用两次add_active_range:
1. add_active_range(0, start, end); // start = 0, end = 57344
2. add_active_range(0, start, end); // start = 196608, end = 262144
/**
* add_active_range - Register a range of PFNs backed by physical memory 【注册物理内存的PFN范围】
* @nid: The node ID the range resides on
* @start_pfn: The start PFN of the available physical memory
* @end_pfn: The end PFN of the available physical memory
* @id : 该物理内存所属的内存结点。由于我们是UMA系统，只用一个内存结点，所以我们在调用时，传入的参数为0.
* @start_pfn： 可用物理内存的开始PFN号
* @end_pfn: 可用屋里内存的结束PFN号
* These ranges are stored in an early_node_map[] and later used by
* free_area_init_nodes() to calculate zone sizes and holes. If the
* range spans a memory hole, it is up to the architecture to ensure
* the memory is not freed by the bootmem allocator. If possible
* the range being registered will be merged with existing ranges.
* 传入的物理内存信息存储到一个数组中：early_node_map[]， 稍后会在 free_area_init_nodes() 函数中使用，用来计算 zone sizes 和 holes。
* 如果物理内存的范围包含一个 memory hole，it is up to the architecture to ensure the memory is not freed by the bootmem allocator.
* 新注册的物理内存范围会Merge到已经存在的物理内存范围中去。
*/
void __init add_active_range(unsigned int nid, unsigned long start_pfn,
unsigned long end_pfn)
{
int i; mminit_dprintk(MMINIT_TRACE, "memory_register",
"Entering add_active_range(%d, %#lx, %#lx) "
"%d entries of %d used\n",
nid, start_pfn, end_pfn,
nr_nodemap_entries, MAX_ACTIVE_REGIONS); mminit_validate_memmodel_limits(&start_pfn, &end_pfn); // 对start_pfn 和 end_pfn 做某些验证，我们不Care /* Merge with existing active regions if possible */
for (i = ; i < nr_nodemap_entries; i++) {
if (early_node_map[i].nid != nid)
continue; /* Skip if an existing region covers this new one */
if (start_pfn >= early_node_map[i].start_pfn &&
end_pfn <= early_node_map[i].end_pfn)
return; /* Merge forward if suitable */
if (start_pfn <= early_node_map[i].end_pfn &&
end_pfn > early_node_map[i].end_pfn) {
early_node_map[i].end_pfn = end_pfn;
return;
} /* Merge backward if suitable */
if (start_pfn < early_node_map[i].start_pfn &&
end_pfn >= early_node_map[i].start_pfn) {
early_node_map[i].start_pfn = start_pfn;
return;
}
} /* Check that early_node_map is large enough */ // earlay_node_map数组有上限
if (i >= MAX_ACTIVE_REGIONS) {
printk(KERN_CRIT "More than %d memory regions, truncating\n",
MAX_ACTIVE_REGIONS);
return;
} early_node_map[i].nid = nid;
early_node_map[i].start_pfn = start_pfn;
early_node_map[i].end_pfn = end_pfn;
nr_nodemap_entries = i + ;
/*
early_node_map[0].nid = 0;
early_node_map[0].start_pfn = 0;
early_node_map[0].end_pfn = 57344;
early_node_map[1].nid = 0;
early_node_map[1].start_pfn = 196608;
early_node_map[1].end_pfn = 262144; 很好理解，不啰嗦了！！！
*/
}
总结：我们在从command_line中确定了屋里内存信息后，这是第一个把这个好消息告诉了buddy system。所以，一定要记住这个关键的数组——>early_node_map.
