init/main.c
#ifdef CONFIG_BLK_DEV_INITRD
kdev_t real_root_dev; 启动参数所设定的根盘设备
#endif
asmlinkage void __init start_kernel(void)
{
char * command_line;
unsigned long mempages;
extern char saved_command_line[];
lock_kernel();
printk(linux_banner);
setup_arch(&command_line); arch/i386/kernel/setup.c中,
初始化initrd_start和initrd_end两个变量
...
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start && !initrd_below_start_ok &&
initrd_start < min_low_pfn << PAGE_SHIFT) {
; min_low_pfn为内核末端_end所开始的物理页号,initrd_start,initrd_end在rd.c中定义
printk(KERN_CRIT "initrd overwritten (0x%08lx < 0x%08lx) - "
"disabling it.\n",initrd_start,min_low_pfn << PAGE_SHIFT);
initrd_start = 0;
}
#endif
...
kernel_thread(init, NULL, CLONE_FS | CLONE_FILES | CLONE_SIGNAL); 创建init进程
unlock_kernel();
current->need_resched = 1;
cpu_idle();
}
static int init(void * unused)
{
lock_kernel();
do_basic_setup();
/*
* Ok, we have completed the initial bootup, and
* we're essentially up and running. Get rid of the
* initmem segments and start the user-mode stuff..
*/
free_initmem();
unlock_kernel();
if (open("/dev/console", O_RDWR, 0) < 0)
printk("Warning: unable to open an initial console.\n");
(void) dup(0);
(void) dup(0);
/*
* We try each of these until one succeeds.
*
* The Bourne shell can be used instead of init if we are
* trying to recover a really broken machine.
*/
if (execute_command)
execve(execute_command,argv_init,envp_init);
execve("/sbin/init",argv_init,envp_init);
execve("/etc/init",argv_init,envp_init);
execve("/bin/init",argv_init,envp_init);
execve("/bin/sh",argv_init,envp_init);
panic("No init found. Try passing init= option to kernel.");
}
static void __init do_basic_setup(void)
{
#ifdef CONFIG_BLK_DEV_INITRD
int real_root_mountflags;
#endif
...
#ifdef CONFIG_BLK_DEV_INITRD
real_root_dev = ROOT_DEV; ROOT_DEV为所请求根文件系统的块设备
real_root_mountflags = root_mountflags;
if (initrd_start && mount_initrd) root_mountflags &= ~MS_RDONLY;
else mount_initrd =0;
#endif
start_context_thread();
do_initcalls(); 会调用partition_setup()中加载内存盘
/* .. filesystems .. */
filesystem_setup();
/* Mount the root filesystem.. */
mount_root();
mount_devfs_fs ();
#ifdef CONFIG_BLK_DEV_INITRD
root_mountflags = real_root_mountflags;
if (mount_initrd && ROOT_DEV != real_root_dev
&& MAJOR(ROOT_DEV) == RAMDISK_MAJOR && MINOR(ROOT_DEV) == 0) {
; 如果当前根盘为initrd所建立的内存盘
int error;
int i, pid;
pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
创建新的任务去执行程序/linuxrc
if (pid>0)
while (pid != wait(&i)); 等待linuxrc进程退出
if (MAJOR(real_root_dev) != RAMDISK_MAJOR
|| MINOR(real_root_dev) != 0) {
; 如果原来的根盘不是0号内存盘,则使用原来的根文件系统,
; 并且将内存盘转移到其/initrd目录下
error = change_root(real_root_dev,"/initrd");
if (error)
printk(KERN_ERR "Change root to /initrd: "
"error %d\n",error);
}
}
#endif
}
#ifdef CONFIG_BLK_DEV_INITRD
static int do_linuxrc(void * shell)
{
static char *argv[] = { "linuxrc", NULL, };
close(0);close(1);close(2);
setsid(); 设置新的session号
(void) open("/dev/console",O_RDWR,0);
(void) dup(0);
(void) dup(0);
return execve(shell, argv, envp_init);
}
#endif
; arch/i386/kernel/setup.c
#define RAMDISK_IMAGE_START_MASK 0x07FF
#define RAMDISK_PROMPT_FLAG 0x8000
#define RAMDISK_LOAD_FLAG 0x4000
#define PARAM ((unsigned char *)empty_zero_page)
#define RAMDISK_FLAGS (*(unsigned short *) (PARAM+0x1F8)) 可用rdev设置的参数
#define LOADER_TYPE (*(unsigned char *) (PARAM+0x210))
#define INITRD_START (*(unsigned long *) (PARAM+0x218)) 初始化盘映象起始物理地址
#define INITRD_SIZE (*(unsigned long *) (PARAM+0x21c)) 初始化盘字节数
void __init setup_arch(char **cmdline_p)
{
...
#ifdef CONFIG_BLK_DEV_RAM
rd_image_start = RAMDISK_FLAGS & RAMDISK_IMAGE_START_MASK; 以块为单位
rd_prompt = ((RAMDISK_FLAGS & RAMDISK_PROMPT_FLAG) != 0);
rd_doload = ((RAMDISK_FLAGS & RAMDISK_LOAD_FLAG) != 0);
#endif
...
#ifdef CONFIG_BLK_DEV_INITRD
if (LOADER_TYPE && INITRD_START) {
if (INITRD_START + INITRD_SIZE <= (max_low_pfn << PAGE_SHIFT)) {
; max_low_pfn表示内核空间1G范围以下最大允许的物理页号
reserve_bootmem(INITRD_START, INITRD_SIZE);
initrd_start =
INITRD_START ? INITRD_START + PAGE_OFFSET : 0; 转变为内核逻辑地址
initrd_end = initrd_start+INITRD_SIZE;
}
else {
printk("initrd extends beyond end of memory "
"(0x%08lx > 0x%08lx)\ndisabling initrd\n",
INITRD_START + INITRD_SIZE,
max_low_pfn << PAGE_SHIFT);
initrd_start = 0;
}
}
#endif
...
}
; fs/partitions/check.c:
int __init partition_setup(void)
{
device_init(); 包含ramdisk设备的初始化
#ifdef CONFIG_BLK_DEV_RAM
#ifdef CONFIG_BLK_DEV_INITRD
if (initrd_start && mount_initrd) initrd_load();
;如果启动时加载了initrd文件,则用它去初始化根内存盘
else
#endif
rd_load(); 如果内核配置了内存盘并且根盘指定为软盘则试图将软盘加载为根内存盘
#endif
return 0;
}
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