上文：http://www.cnblogs.com/long123king/p/3549701.html
1: static int __init kernel_init(void * unused)
2: { 3: ...... 4: 5: if (!ramdisk_execute_command)
6: ramdisk_execute_command = "/init";
7: 8: if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
9: ramdisk_execute_command = NULL; 10: prepare_namespace(); 11: } 12: 13: /*
14: * Ok, we have completed the initial bootup, and
15: * we're essentially up and running. Get rid of the
16: * initmem segments and start the user-mode stuff..
17: */
18: 19: init_post();20: return 0;
21: }如何在Source Insight中查找sys_access在哪里，直接找是找不到了，不如通过”Ctrl + /”来全局查找，查找：
1: SYSCALL_DEFINE2(access这里面是对/init做一下检测，看看是否存在该文件，sys_access用来检测是否有权限访问该文件。
返回值不为0，代表无法访问该文件，就执行prepare_namespace另作准备。
如果这里可以访问/init文件，那么就直接调用该文件来进行初始化设置。
因此，可以推测，prepare_namespace与initrd中的/init的作用类似。
1: /*
2: * Prepare the namespace - decide what/where to mount, load ramdisks, etc.
3: */
4: void __init prepare_namespace(void)
5: {注释的意思，决定挂载点，加载内存盘。
1: if (root_delay) {
2: printk(KERN_INFO "Waiting %dsec before mounting root device... ",
3: root_delay); 4: ssleep(root_delay); 5: }是用来在挂载根文件系统之前，等待某些比较慢的IO设备准备好根设备。
root_device_name就是我们在grub中通过root=/dev/sda1设置的根文件设备。
1: static int __init root_dev_setup(char *line)
2: {3: strlcpy(saved_root_name, line, sizeof(saved_root_name));
4: return 1;
5: } 6: 7: __setup("root=", root_dev_setup);
1: if (saved_root_name[0]) {
2: root_device_name = saved_root_name;3: if (!strncmp(root_device_name, "mtd", 3) ||
4: !strncmp(root_device_name, "ubi", 3)) {
5: mount_block_root(root_device_name, root_mountflags);6: goto out;
7: } 8: ROOT_DEV = name_to_dev_t(root_device_name);9: if (strncmp(root_device_name, "/dev/", 5) == 0)
10: root_device_name += 5; 11: }对于root=/dev/sda1，将root_device_name设置成sda1。
ROOT_DEV = name_to_dev_t(root_device_name);
在这之前，已经获取到了ROOT_DEV的描述符。
1: if (initrd_load())
2: goto out;
3: 4: /* wait for any asynchronous scanning to complete */
5: if ((ROOT_DEV == 0) && root_wait) {
6: printk(KERN_INFO "Waiting for root device %s... ",
7: saved_root_name);8: while (driver_probe_done() != 0 ||
9: (ROOT_DEV = name_to_dev_t(saved_root_name)) == 0) 10: msleep(100); 11: async_synchronize_full(); 12: } 13: 14: is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR; 15: 16: if (is_floppy && rd_doload && rd_load_disk(0))
17: ROOT_DEV = Root_RAM0; 18: 19: mount_root(); 20: 21: devtmpfs_mount("dev");
22: sys_mount(".", "/", NULL, MS_MOVE, NULL);
23: sys_chroot((const char __user __force *)".");
如果initrd_load()成功了，就跳过mount_root这一步，这表明initrd_load()如果成功，就已经将root根文件系统给挂载好了。
否则，调用mount_root/mount_block_root，尝试以各种可能的文件系统挂载根文件系统
1: static char * __initdata root_fs_names;
2: static int __init fs_names_setup(char *str)
3: { 4: root_fs_names = str;5: return 1;
6: } 7: 8: static unsigned int __initdata root_delay;
9: 10: __setup("rootfstype=", fs_names_setup);
1: static int __init do_mount_root(char *name, char *fs, int flags, void *data)
2: {3: int err = sys_mount(name, "/root", fs, flags, data);
4: if (err)
5: return err;
6: 7: sys_chdir((const char __user __force *)"/root");
8: ROOT_DEV = current->fs->pwd.mnt->mnt_sb->s_dev; 9: printk(KERN_INFO10: "VFS: Mounted root (%s filesystem)%s on device %u:%u. ",
11: current->fs->pwd.mnt->mnt_sb->s_type->name, 12: current->fs->pwd.mnt->mnt_sb->s_flags & MS_RDONLY ?13: " readonly" : "", MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
14: return 0;
15: }因此，这里的错误在于，没有正确地执行initrd中的初始化程序，也没有成功地mount根文件系统。
明明是ext2的根文件系统，为什么没有被mount上呢？这是问题的根本原因。