一、devtmpfs概述
1.devtmpfs 的功用是在 Linux 核心 启动早期建立一个初步的 /dev，令一般启动程序不用等待 udev，缩短 GNU/Linux 的开机时间。
2.重要解释
Devtmpfs lets the kernel create a tmpfs very early at kernel initialization, before any driver core device is registered. Every device with a major/minor will have a device node created in this tmpfs instance. After the rootfs is mounted by the kernel, the populated tmpfs is mounted at /dev. In initramfs, it can be moved to the manually mounted root filesystem before /sbin/init is executed.
3.menuconfig 中加入devtmpfs支持
make menuconfig-->Device Drivers-->Generic Driver Options
Maintain a devtmpfs filesystem to mount at /dev
Automount devtmpfs at /dev, after the kernel mounted the rootfs
4.df -T显示devtmpfs
文件系统 类型 1K-块 已用 可用 已用% 挂载点
/dev/sda1 ext4 31621016 14985712 15029008 50% /
none devtmpfs 399552 276 399276 1% /dev
none tmpfs 403804 24 403780 1% /dev/shm
none tmpfs 403804 108 403696 1% /var/run
none tmpfs 403804 0 403804 0% /var/lock
none tmpfs 403804 0 403804 0% /lib/init/rw
.host:/ vmhgfs 67151668 54038400 13113268 81% /mnt/hgfs
/dev/loop0 ext2 16119 8528 6772 56% /mnt/loop
二、devtmpfs文件系统初始化
void __init driver_init(void)
{
/* These are the core pieces */
devtmpfs_init();//devtmpfs文件系统初始化
devices_init();
buses_init();
classes_init();
firmware_init();
hypervisor_init();
platform_bus_init();
system_bus_init();
cpu_dev_init();
memory_dev_init();
}
static struct file_system_type dev_fs_type = {
.name = "devtmpfs",
.mount = dev_mount,
.kill_sb = kill_litter_super,
};
int __init devtmpfs_init(void)
{
int err = register_filesystem(&dev_fs_type);//注册dev_fs_type文件系统，即将dev_fs_type添加到内核全局总链表中file_systems
if (err) {
printk(KERN_ERR "devtmpfs: unable to register devtmpfs ""type %i\n", err);
return err;
}
thread = kthread_run(devtmpfsd, &err, "kdevtmpfs");//创建并启动一个内核线程devtmpfsd
if (!IS_ERR(thread)) {
wait_for_completion(&setup_done);//进行一个不可打断的等待,允许一个线程告诉另一个线程工作已经完成
} else {
err = PTR_ERR(thread);
thread = NULL;
}
if (err) {
printk(KERN_ERR "devtmpfs: unable to create devtmpfs %i\n", err);
unregister_filesystem(&dev_fs_type);
return err;
}
printk(KERN_INFO "devtmpfs: initialized\n");
return 0;
}
//请求创建设备节点的请求队列req结构
static struct req {
struct req *next;
struct completion done;
int err;
const char *name;
umode_t mode;//0代表删除
struct device *dev;
} *requests;
//内核线程devtmpfsd
static int devtmpfsd(void *p)
{
char options[] = "mode=0755";
int *err = p;
*err = sys_unshare(CLONE_NEWNS);
if (*err)
goto out;
//挂载devtmpfs文件系统
//devtmpfs是待安装设备的路径名，“/”是安装点路径名，”devtmpfs“表示文件系统类型，MS_SILENT=32768，即0x8000
*err = sys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
if (*err)
goto out;
sys_chdir("/.."); //将进程的当前工作目录(pwd)设定为devtmpfs文件系统的根目录/* will traverse into overmounted root */
sys_chroot(".");
complete(&setup_done);//允许一个线程告诉另一个线程工作已经完成
while (1) {
spin_lock(&req_lock);
while (requests) {//扫描请求链表，每当要创建一个设备节点时，都需要向requests链表中添加请求
struct req *req = requests;//赋值给临时req
requests = NULL;//清空
spin_unlock(&req_lock);
while (req) {//遍历刚才requests的请求链表
struct req *next = req->next;
req->err = handle(req->name, req->mode, req->dev);//对链表中的每一个请求调用handle函数
complete(&req->done);
req = next;
}
spin_lock(&req_lock);
}
__set_current_state(TASK_INTERRUPTIBLE);//设置为睡眠状态
spin_unlock(&req_lock);
schedule();//系统切换
}
return 0;
out:
complete(&setup_done);
return *err;
}
static int handle(const char *name, umode_t mode, struct device *dev)
{
if (mode)
return handle_create(name, mode, dev);
else
return handle_remove(name, dev);
}
static int handle_create(const char *nodename, umode_t mode, struct device *dev)
{
struct dentry *dentry;
struct path path;
int err;
//查找节点名称的路径以及返回节点对应的父目录dentry结构，即在此目录下创建一个设备节点，即是/dev目录对应的dentry结构
dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
if (dentry == ERR_PTR(-ENOENT)) {
create_path(nodename);
dentry = kern_path_create(AT_FDCWD, nodename, &path, 0);
}
if (IS_ERR(dentry))
return PTR_ERR(dentry);
//创建设备节点
err = vfs_mknod(path.dentry->d_inode,dentry, mode, dev->devt);
if (!err) {
struct iattr newattrs;
newattrs.ia_mode = mode;/* fixup possibly umasked mode */
newattrs.ia_valid = ATTR_MODE;
mutex_lock(&dentry->d_inode->i_mutex);
notify_change(dentry, &newattrs);
mutex_unlock(&dentry->d_inode->i_mutex);
dentry->d_inode->i_private = &thread;/* mark as kernel-created inode */
}
done_path_create(&path, dentry);//与前边kern_path_create对应，减少path和dentry的计数等
return err;
}
int vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
int error = may_create(dir, dentry);//检查是否可以创建设备文件节点
if (error)
return error;
//必须是字符设备或者块设备，且具有创建节点的权限
if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
return -EPERM;
if (!dir->i_op->mknod)
return -EPERM;
error = devcgroup_inode_mknod(mode, dev);
if (error)
return error;
error = security_inode_mknod(dir, dentry, mode, dev);
if (error)
return error;
//调用具体文件系统的mknod()函数
//mount时调用shmem_fill_super()-->shmem_get_inode()分配inode节点时做出的初始化
/*那么在shmem_get_inode中
caseS_IFDIR:
inc_nlink(inode);
inode->i_size= 2 * BOGO_DIRENT_SIZE;
inode->i_op= &shmem_dir_inode_operations;
inode->i_fop= &simple_dir_operations;
由于mountpoint是dev这个目录，所以dev对应的inode的i_op就是shmem_dir_inode_operations。
staticconst struct inode_operations shmem_dir_inode_operations = {
#ifdefCONFIG_TMPFS
.create =shmem_create,
.lookup =simple_lookup,
.link =shmem_link,
.unlink =shmem_unlink,
.symlink =shmem_symlink,
.mkdir =shmem_mkdir,
.rmdir =shmem_rmdir,
.mknod =shmem_mknod,
.rename =shmem_rename,
#endif
#ifdefCONFIG_TMPFS_POSIX_ACL
.setattr =shmem_notify_change,
.setxattr =generic_setxattr,
.getxattr =generic_getxattr,
.listxattr =generic_listxattr,
.removexattr =generic_removexattr,
.check_acl =generic_check_acl,
#endif
};
*/
error = dir->i_op->mknod(dir, dentry, mode, dev);//所以这里调用的就是shmem_mknod
if (!error)
fsnotify_create(dir, dentry);
return error;
}
shmem_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
struct inode *inode;
int error = -ENOSPC;
inode = shmem_get_inode(dir->i_sb, dir, mode, dev, VM_NORESERVE);//获得一个要创建的设备节点的inode，并初始化
if (inode) {
error = security_inode_init_security(inode, dir,&dentry->d_name,shmem_initxattrs, NULL);
if (error) {
if (error != -EOPNOTSUPP) {
iput(inode);
return error;
}
}
#ifdef CONFIG_TMPFS_POSIX_ACL
error = generic_acl_init(inode, dir);
if (error) {
iput(inode);
return error;
}
#else
error = 0;
#endif
dir->i_size += BOGO_DIRENT_SIZE;
dir->i_ctime = dir->i_mtime = CURRENT_TIME;
d_instantiate(dentry, inode);//与dentry建立关,此时就可以在/dev下看到这个字符设备节点了
dget(dentry); //递减dentry的计数
}
return error;
}
三、文件系统的mount
内核主要是通过kernel_init调用prepare_namespace()函数执行安装实际根文件系统的操作：
void __init prepare_namespace(void)
{
int is_floppy;
if (root_delay) {
printk(KERN_INFO "Waiting %dsec before mounting root device...\n",
root_delay);
ssleep(root_delay);
}
wait_for_device_probe();
md_run_setup();
/* 把root_device_name变量置为从启动参数“root”中获取的设备文件名。
* 同样，把ROOT_DEV变量置为同一设备文件的主设备号和次设备号。*/
if (saved_root_name[0]) {
root_device_name = saved_root_name;
if (!strncmp(root_device_name, "mtd", 3) ||
!strncmp(root_device_name, "ubi", 3)) {
mount_block_root(root_device_name, root_mountflags);
goto out;
}
ROOT_DEV = name_to_dev_t(root_device_name);//转换为设备号/dev/mtdblock2.
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);
async_synchronize_full();
}
is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR;
if (is_floppy && rd_doload && rd_load_disk(0))
ROOT_DEV = Root_RAM0;
mount_root();
out:
devtmpfs_mount("dev");//挂载devtmpfs文件系统
sys_mount(".", "/", NULL, MS_MOVE, NULL); /* 移动rootfs文件系统根目录上的已安装文件系统的安装点。 */
sys_chroot(".");
}
int devtmpfs_mount(const char *mntdir)
{
int err;
if (!mount_dev)
return 0;
if (!thread)
return 0;
//将devtmpfs文件系统挂载到/dev目录下
err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT, NULL);
if (err)
printk(KERN_INFO "devtmpfs: error mounting %i\n", err);
else
printk(KERN_INFO "devtmpfs: mounted\n");
return err;
}
四、devtmpfs创建节点
系统在启动过程中，扫描到的设备会通过devtmpfs_create_node()函数来添加设备节点。
int devtmpfs_create_node(struct device *dev)
{
const char *tmp = NULL;
struct req req;
if (!thread)
return 0;
req.mode = 0;
req.name = device_get_devnode(dev, &req.mode, &tmp);//获得设备名
if (!req.name)
return -ENOMEM;
if (req.mode == 0)
req.mode = 0600;
if (is_blockdev(dev))
req.mode |= S_IFBLK;//块设备
else
req.mode |= S_IFCHR;//字符设备
req.dev = dev;
init_completion(&req.done);
spin_lock(&req_lock);
req.next = requests;//请求添加到requests链表
requests = &req;
spin_unlock(&req_lock);
wake_up_process(thread);//唤醒内核线程devtmpfsd添加设备节点
wait_for_completion(&req.done);
kfree(tmp);
return req.err;
}
const char *device_get_devnode(struct device *dev,umode_t *mode, const char **tmp)
{
char *s;
*tmp = NULL;
/* the device type may provide a specific name */
if (dev->type && dev->type->devnode)
*tmp = dev->type->devnode(dev, mode);
if (*tmp)
return *tmp;
/* the class may provide a specific name */
if (dev->class && dev->class->devnode)
*tmp = dev->class->devnode(dev, mode);
if (*tmp)
return *tmp;
/* return name without allocation, tmp == NULL */
if (strchr(dev_name(dev), '!') == NULL)
return dev_name(dev);
/* replace '!' in the name with '/' */
*tmp = kstrdup(dev_name(dev), GFP_KERNEL);
if (!*tmp)
return NULL;
while ((s = strchr(*tmp, '!')))
s[0] = '/';
return *tmp;
}
|
