/*
* 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..
*/
init_post();
return 0;
}
ramdisk_execute_command:在kernel解析引导参数的时候使用。如果用户指定了init文件路径，即使用了“init=”，就会将这个参数值存放到这里。
如果没有指定init文件路径。默认为/init
对应于前面一段的分析，我们知道，对于initramdisk和cpio-initrd的情况，都会将虚拟根文件系统释放到根目录。如果这些虚拟文件系统里有/init这个文件。就会转入到init_post()。
Init_post()代码如下：
static int noinline init_post(void) //init/main.c
{
free_initmem();
unlock_kernel();
mark_rodata_ro();
system_state = SYSTEM_RUNNING;
numa_default_policy();
if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
printk(KERN_WARNING "Warning: unable to open an initial console.\n");
(void) sys_dup(0);
(void) sys_dup(0);
if (ramdisk_execute_command) {
run_init_process(ramdisk_execute_command);
printk(KERN_WARNING "Failed to execute %s\n",
ramdisk_execute_command);
}
/*
* 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) {
run_init_process(execute_command);
printk(KERN_WARNING "Failed to execute %s. Attempting "
"defaults...\n", execute_command);
}
run_init_process("/sbin/init");
run_init_process("/etc/init");
run_init_process("/bin/init");
run_init_process("/bin/sh");
panic("No init found. Try passing init= option to kernel.");
}
从代码中可以看中，会依次执行指定的init文件，如果失败，就会执行/sbin/init, /etc/init,, /bin/init,/bin/sh
注意的是，run_init_process在调用相应程序运行的时候，用的是kernel_execve。也就是说调用进程会替换当前进程。只要上述任意一个文件调用成功，就不会返回到这个函数。如果上面几个文件都无法执行。打印出没有找到init文件的错误。
对于image-hdr或者是虚拟文件系统中没有包含 /init的情况，会由prepare_namespace()处理。代码如下：
void __init prepare_namespace(void)
//init/do_mount.c
{
int is_floppy;
if (root_delay) {
printk(KERN_INFO "Waiting %dsec before mounting root device...\n",
root_delay);
ssleep(root_delay);
}
/* wait for the known devices to complete their probing */
while (driver_probe_done() != 0)
msleep(100);
//mtd的处理
md_run_setup();
if (saved_root_name[0]) {
root_device_name = saved_root_name;
if (!strncmp(root_device_name, "mtd", 3)) {
mount_block_root(root_device_name, root_mountflags);
goto out;
}
ROOT_DEV = name_to_dev_t(root_device_name);
if (strncmp(root_device_name, "/dev/", 5) == 0)
root_device_name += 5;
}
if (initrd_load())
goto out;
/* wait for any asynchronous scanning to complete */
if ((ROOT_DEV == 0) && root_wait) {
printk(KERN_INFO "Waiting for root device %s...\n",
saved_root_name);
while (driver_probe_done() != 0 ||
(ROOT_DEV = name_to_dev_t(saved_root_name)) == 0)
msleep(100);
}
is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR;
if (is_floppy && rd_doload && rd_load_disk(0))
ROOT_DEV = Root_RAM0;
mount_root();
out:
sys_mount(".", "/", NULL, MS_MOVE, NULL);
sys_chroot(".");
}
这里有几个比较有意思的处理，首先用户可以用root=来指定根文件系统。它的值保存在saved_root_name中。如果用户指定了以mtd开始的字串做为它的根文件系统。就会直接去挂载。这个文件是mtdblock的设备文件。
否则将设备结点文件转换为ROOT_DEV即设备节点号
然后，转向initrd_load()执行initrd预处理后，再将具体的根文件系统挂载。
注意到，在这个函数末尾。会调用sys_mount()来移动当前文件系统挂载点到”/”目录下。然后将根目录切换到当前目录。这样，根文件系统的挂载点就成为了我们在用户空间所看到的”/”了.
对于其它根文件系统的情况，会先经过initrd的处理。即
int __init initrd_load(void) //init/do_mounts_initrd.c
{
if (mount_initrd) {
create_dev("/dev/ram", Root_RAM0);
/*
* Load the initrd data into /dev/ram0. Execute it as initrd
* unless /dev/ram0 is supposed to be our actual root device,
* in that case the ram disk is just set up here, and gets
* mounted in the normal path.
*/
if (rd_load_image("/initrd.image") && ROOT_DEV != Root_RAM0) {
sys_unlink("/initrd.image");
handle_initrd();
return 1;
}
}
sys_unlink("/initrd.image");
return 0;
}
建立一个ROOT_RAM)的设备节点，并将/initrd/.image释放到这个节点中，/initrd.image的内容，就是我们之前分析的image-initrd。
如果根文件设备号不是ROOT_RAM0( 用户指定的根文件系统不是/dev/ram0就会转入到handle_initrd()
如果当前根文件系统是/dev/ram0.将其直接挂载就好了。
handle_initrd()代码如下：
static void __init handle_initrd(void)
//init/do_mounts_initrd.c
{
int error;
int pid;
real_root_dev = new_encode_dev(ROOT_DEV);
create_dev("/dev/root.old", Root_RAM0);
/* mount initrd on rootfs' /root */
mount_block_root("/dev/root.old", root_mountflags & ~MS_RDONLY);
sys_mkdir("/old", 0700);
root_fd = sys_open("/", 0, 0);
old_fd = sys_open("/old", 0, 0);
/* move initrd over / and chdir/chroot in initrd root */
sys_chdir("/root");
sys_mount(".", "/", NULL, MS_MOVE, NULL);
sys_chroot(".");
/*
* In case that a resume from disk is carried out by linuxrc or one of
* its children, we need to tell the freezer not to wait for us.
*/
current->flags |= PF_FREEZER_SKIP;
pid = kernel_thread(do_linuxrc, "/linuxrc", SIGCHLD);
if (pid > 0)
while (pid != sys_wait4(-1, NULL, 0, NULL))
yield();
current->flags &= ~PF_FREEZER_SKIP;
/* move initrd to rootfs' /old */
sys_fchdir(old_fd);
sys_mount("/", ".", NULL, MS_MOVE, NULL);
/* switch root and cwd back to / of rootfs */
sys_fchdir(root_fd);
sys_chroot(".");
sys_close(old_fd);
sys_close(root_fd);
if (new_decode_dev(real_root_dev) == Root_RAM0) {
sys_chdir("/old");
return;
}
ROOT_DEV = new_decode_dev(real_root_dev);
mount_root();
printk(KERN_NOTICE "Trying to move old root to /initrd ... ");
error = sys_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
if (!error)
printk("okay\n");
else {
int fd = sys_open("/dev/root.old", O_RDWR, 0);
if (error == -ENOENT)
printk("/initrd does not exist. Ignored.\n");
else
printk("failed\n");
printk(KERN_NOTICE "Unmounting old root\n");
sys_umount("/old", MNT_DETACH);
printk(KERN_NOTICE "Trying to free ramdisk memory ... ");
if (fd < 0) {
error = fd;
} else {
error = sys_ioctl(fd, BLKFLSBUF, 0);
sys_close(fd);
}
printk(!error ? "okay\n" : "failed\n");
}
}
先将/dev/ram0挂载，而后执行/linuxrc.等其执行完后。切换根目录，再挂载具体的根文件系统.
到这里。文件系统挂载的全部内容就分析完了.
五：小结
在本小节里。分析了根文件系统的挂载流程。并对几个虚拟根文件系统的情况做了详细的分析。理解这部份，对我们构建linux嵌入式开发系统是很有帮助的.