linux mount过程

接下来我们以ext4 文件系统mount过程为例，讲解下文件系统的
几种数据结构之间的关联。

如果linux版本有支持ext4 fs，那么在linux初始化时会调用static int __init ext4_init_fs(void)，
这个函数会通过register_filesystem(&ext4_fs_type)向系统注册ext4文件系统到全局file_systems结构中。
注册之后os就可以识别此文件系统，当要使用ext4时，通过mount函数加载ext4 super block,inode信息，

之后就可以进行ext4读写了。

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,char __user *, type, unsigned long, flags, void __user *, data){int ret;char *kernel_type;struct filename *kernel_dir;char *kernel_dev;unsigned long data_page;//复制文件系统类型名到内核ret = copy_mount_string(type, &kernel_type);if (ret < 0)goto out_type;//得到挂载点路径名kernel_dir = getname(dir_name);if (IS_ERR(kernel_dir)) {ret = PTR_ERR(kernel_dir);goto out_dir;}//获取文件系统所在设备名，如/dev/sda1ret = copy_mount_string(dev_name, &kernel_dev);if (ret < 0)goto out_dev;//获取挂载options信息ret = copy_mount_options(data, &data_page);if (ret < 0)goto out_data;//mount主体函数ret = do_mount(kernel_dev, kernel_dir->name, kernel_type, flags,(void *) data_page);free_page(data_page);out_data:kfree(kernel_dev);out_dev:putname(kernel_dir);out_dir:kfree(kernel_type);out_type:return ret;}

//先检查挂载参数，之后调用不同的mount函数long do_mount(const char *dev_name, const char *dir_name,const char *type_page, unsigned long flags, void *data_page){struct path path;int retval = 0;int mnt_flags = 0;printk(KERN_ERR "dev_name:%s dir_name:%s \n",dev_name,dir_name);/* Discard magic */if ((flags & MS_MGC_MSK) == MS_MGC_VAL)flags &= ~MS_MGC_MSK;/* Basic sanity checks */if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))return -EINVAL;if (data_page)((char *)data_page)[PAGE_SIZE - 1] = 0;/* ... and get the mountpoint *///解析dir_name获取挂载路径retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);if (retval)return retval;//挂载安全性检测retval = security_sb_mount(dev_name, &path,   type_page, flags, data_page);if (!retval && !may_mount())retval = -EPERM;if (retval)goto dput_out;/* Default to relatime unless overriden */if (!(flags & MS_NOATIME))mnt_flags |= MNT_RELATIME;/* Separate the per-mountpoint flags */if (flags & MS_NOSUID)mnt_flags |= MNT_NOSUID;if (flags & MS_NODEV)mnt_flags |= MNT_NODEV;if (flags & MS_NOEXEC)mnt_flags |= MNT_NOEXEC;if (flags & MS_NOATIME)mnt_flags |= MNT_NOATIME;if (flags & MS_NODIRATIME)mnt_flags |= MNT_NODIRATIME;if (flags & MS_STRICTATIME)mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);if (flags & MS_RDONLY)mnt_flags |= MNT_READONLY;/* The default atime for remount is preservation */if ((flags & MS_REMOUNT) &&    ((flags & (MS_NOATIME | MS_NODIRATIME | MS_RELATIME |       MS_STRICTATIME)) == 0)) {mnt_flags &= ~MNT_ATIME_MASK;mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;}flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |   MS_STRICTATIME);if (flags & MS_REMOUNT)retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,    data_page);else if (flags & MS_BIND)retval = do_loopback(&path, dev_name, flags & MS_REC);else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))retval = do_change_type(&path, flags);else if (flags & MS_MOVE)retval = do_move_mount(&path, dev_name);elseretval = do_new_mount(&path, type_page, flags, mnt_flags,      dev_name, data_page);dput_out:path_put(&path);return retval;}/*对于一个新的文件系统初次挂载会调用do_new_mount，这个函数会先给这个文件系统创建一个struct mount结构，调用文件系统特有的mount函数，最后将struct mount加入到全局文件树中*/static int do_new_mount(struct path *path, const char *fstype, int flags,int mnt_flags, const char *name, void *data){struct file_system_type *type;struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;struct vfsmount *mnt;int err;if (!fstype)return -EINVAL;//通过name获取文件系统类型type = get_fs_type(fstype);if (!type)return -ENODEV;printk(KERN_ERR "fs type:%s\n",type->name);if (user_ns != &init_user_ns) {if (!(type->fs_flags & FS_USERNS_MOUNT)) {put_filesystem(type);return -EPERM;}/* Only in special cases allow devices from mounts * created outside the initial user namespace. */if (!(type->fs_flags & FS_USERNS_DEV_MOUNT)) {flags |= MS_NODEV;mnt_flags |= MNT_NODEV | MNT_LOCK_NODEV;}}//获取struct mount结构,调用特定文件系统mount函数，主要填充super block数据mnt = vfs_kern_mount(type, flags, name, data);//有子文件系统if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&    !mnt->mnt_sb->s_subtype)mnt = fs_set_subtype(mnt, fstype);put_filesystem(type);if (IS_ERR(mnt))return PTR_ERR(mnt);//将mount加入到全局文件树中err = do_add_mount(real_mount(mnt), path, mnt_flags);if (err)mntput(mnt);return err;}struct vfsmount *vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data){struct mount *mnt;struct dentry *root;if (!type)return ERR_PTR(-ENODEV);//分配并初始化struct mount 结构mnt = alloc_vfsmnt(name);if (!mnt)return ERR_PTR(-ENOMEM);if (flags & MS_KERNMOUNT)mnt->mnt.mnt_flags = MNT_INTERNAL;//调用具体文件系统的mount函数root = mount_fs(type, flags, name, data);if (IS_ERR(root)) {free_vfsmnt(mnt);return ERR_CAST(root);}//初始化mnt变量，并将mnt加入超级块s_mounts链表中mnt->mnt.mnt_root = root;mnt->mnt.mnt_sb = root->d_sb;mnt->mnt_mountpoint = mnt->mnt.mnt_root;mnt->mnt_parent = mnt;br_write_lock(&vfsmount_lock);list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);br_write_unlock(&vfsmount_lock);return &mnt->mnt;}struct dentry *mount_fs(struct file_system_type *type, int flags, const char *name, void *data){struct dentry *root;struct super_block *sb;char *secdata = NULL;int error = -ENOMEM;if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {secdata = alloc_secdata();if (!secdata)goto out;error = security_sb_copy_data(data, secdata);if (error)goto out_free_secdata;}//具体文件系统的mount函数，比如ext4，该函数就是系统初始化时注册的ext4_fs_type 里面的mountroot = type->mount(type, flags, name, data);//返回mount后的dentyif (IS_ERR(root)) {error = PTR_ERR(root);goto out_free_secdata;}sb = root->d_sb;BUG_ON(!sb);WARN_ON(!sb->s_bdi);WARN_ON(sb->s_bdi == &default_backing_dev_info);sb->s_flags |= MS_BORN;error = security_sb_kern_mount(sb, flags, secdata);if (error)goto out_sb;/* * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE * but s_maxbytes was an unsigned long long for many releases. Throw * this warning for a little while to try and catch filesystems that * violate this rule. */WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to ""negative value (%lld)\n", type->name, sb->s_maxbytes);up_write(&sb->s_umount);free_secdata(secdata);return root;out_sb:dput(root);deactivate_locked_super(sb);out_free_secdata:free_secdata(secdata);out:return ERR_PTR(error);}//newmnt: 新创建的挂载实例 path:挂载路径static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags){struct mountpoint *mp;struct mount *parent;int err;mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);    //这里不是简单的加锁，如果path上挂载了很多文件系统，那么这里就是要找出最新一次挂载到其上的文件系统的根路径，这才//是我们这个文件系统要挂载到的mountpointmp = lock_mount(path);if (IS_ERR(mp))return PTR_ERR(mp);parent = real_mount(path->mnt);//得到挂载点所属的挂载结构err = -EINVAL;if (unlikely(!check_mnt(parent))) {/* that's acceptable only for automounts done in private ns */if (!(mnt_flags & MNT_SHRINKABLE))goto unlock;/* ... and for those we'd better have mountpoint still alive */if (!parent->mnt_ns)goto unlock;}/* Refuse the same filesystem on the same mount point */err = -EBUSY;//禁止同一个文件系统挂在到同一个挂载点if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&    path->mnt->mnt_root == path->dentry)goto unlock;err = -EINVAL;if (S_ISLNK(newmnt->mnt.mnt_root->d_inode->i_mode))goto unlock;newmnt->mnt.mnt_flags = mnt_flags;//把newmnt加入到全局文件系统树中err = graft_tree(newmnt, parent, mp);  unlock:unlock_mount(mp);return err;}static struct mountpoint *lock_mount(struct path *path){struct vfsmount *mnt;struct dentry *dentry = path->dentry;retry:mutex_lock(&dentry->d_inode->i_mutex);if (unlikely(cant_mount(dentry))) {mutex_unlock(&dentry->d_inode->i_mutex);return ERR_PTR(-ENOENT);}namespace_lock();mnt = lookup_mnt(path);if (likely(!mnt)) {//这里表示dentry上未挂载文件系统，创建一个新的mountpoint 返回struct mountpoint *mp = new_mountpoint(dentry);if (IS_ERR(mp)) {namespace_unlock();mutex_unlock(&dentry->d_inode->i_mutex);return mp;}return mp;}namespace_unlock();mutex_unlock(&path->dentry->d_inode->i_mutex);path_put(path); // 如果lookup_mnt没有返回NULL，则说明它找到了挂载在/mnt上的子文件系统，下面的逻辑是：  // 把子文件系统的mount结构赋值给path->mnt path->mnt = mnt;//如果此dentry之前挂载了文件系统，则新的dentry将为子文件系统mnt的挂载点dentry = path->dentry = dget(mnt->mnt_root);// 返回到lookup_mnt函数，用新的path变量继续查找是否还有后续的子文件系统//这样组成的list结构:p->C1->C2->C3，从全局来看后挂载的会覆盖之前挂载的文件系统goto retry;}//参数为挂载点所属的挂载实例跟目录项,dir为移动方向/*路径名查找时都会调用到这个函数，它的作用就是根据一个父<mount, dentry>二元组找到挂载在其下面的子文件系统的mount实例，如果没找到就返回NULL*/struct mount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,      int dir){struct list_head *head = mount_hashtable + hash(mnt, dentry);struct list_head *tmp = head;struct mount *p, *found = NULL;for (;;) {tmp = dir ? tmp->next : tmp->prev;p = NULL;if (tmp == head)//循环一圈未找到break;p = list_entry(tmp, struct mount, mnt_hash);//mnt_hash 链接到mount_hashtableif (&p->mnt_parent->mnt == mnt && p->mnt_mountpoint == dentry) {//p其实是参数mnt的子文件系统found = p;break;}}return found;}