EXT4文件系统学习（15）VFS之VFS 文件/目录对象

本文主要是介绍EXT4文件系统学习（15）VFS之VFS 文件/目录对象，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

无论下层具体文件系统差异如何，VFS通过file结构向上层提供一个统一的文件目录对象。

VFS的文件对象

struct file {union {struct llist_node	fu_llist;struct rcu_head 	fu_rcuhead;} f_u;struct path		f_path;struct inode		*f_inode;	/* cached value */const struct file_operations	*f_op; 文件操作函数指针/** Protects f_ep_links, f_flags.* Must not be taken from IRQ context.*/spinlock_t		f_lock;atomic_long_t		f_count; 引用计数unsigned int 		f_flags;fmode_t			f_mode;struct mutex		f_pos_lock;loff_t			f_pos; 文件的读写指针struct fown_struct	f_owner;const struct cred	*f_cred;struct file_ra_state	f_ra;u64			f_version;
#ifdef CONFIG_SECURITYvoid			*f_security;
#endif/* needed for tty driver, and maybe others */void			*private_data;#ifdef CONFIG_EPOLL/* Used by fs/eventpoll.c to link all the hooks to this file */struct list_head	f_ep_links;struct list_head	f_tfile_llink;
#endif /* #ifdef CONFIG_EPOLL */struct address_space	*f_mapping;
} __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */

比较重要的是f_op指针，指向一个file_operations结构体，定义如下：

struct file_operations {struct module *owner;loff_t (*llseek) (struct file *, loff_t, int);ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);int (*iterate) (struct file *, struct dir_context *);unsigned int (*poll) (struct file *, struct poll_table_struct *);long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);long (*compat_ioctl) (struct file *, unsigned int, unsigned long);int (*mmap) (struct file *, struct vm_area_struct *);int (*mremap)(struct file *, struct vm_area_struct *);int (*open) (struct inode *, struct file *);int (*flush) (struct file *, fl_owner_t id);int (*release) (struct inode *, struct file *);int (*fsync) (struct file *, loff_t, loff_t, int datasync);int (*aio_fsync) (struct kiocb *, int datasync);int (*fasync) (int, struct file *, int);int (*lock) (struct file *, int, struct file_lock *);ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);int (*check_flags)(int);int (*flock) (struct file *, int, struct file_lock *);ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);int (*setlease)(struct file *, long, struct file_lock **, void **);long (*fallocate)(struct file *file, int mode, loff_t offset,loff_t len);void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMUunsigned (*mmap_capabilities)(struct file *);
#endif
};

read和write是同步阻塞读写函数，read_iter和write_iter是异步读写，由于需要兼容下层大多数的具体文件系统，所有定义了所有函数指针，但是在ext4_file_operations中只对某些函数指针赋值，file对象的f_op指针类型与inode的i_fop是一致的，i_fop是在ext4_fill_super函数中inode初始化时赋值的，f_op应该是也是在file对象初始化赋值的。

VFS的目录对象

在VFS中每个目录项都对应一个dentry对象，dentry对象是下层具体文件系统目录项的抽象，定义如下：

struct dentry {/* RCU lookup touched fields */unsigned int d_flags;		/* protected by d_lock */seqcount_t d_seq;		/* per dentry seqlock */struct hlist_bl_node d_hash;	/* lookup hash list */struct dentry *d_parent;	/* parent directory */struct qstr d_name; 目录名struct inode *d_inode;		/* Where the name belongs to - NULL is 目录对应inode* negative */unsigned char d_iname[DNAME_INLINE_LEN];	/* small names *//* Ref lookup also touches following */struct lockref d_lockref;	/* per-dentry lock and refcount */const struct dentry_operations *d_op; 目录操作函数指针struct super_block *d_sb;	/* The root of the dentry tree */unsigned long d_time;		/* used by d_revalidate */void *d_fsdata;			/* fs-specific data */struct list_head d_lru;		/* LRU list */struct list_head d_child;	/* child of parent list */struct list_head d_subdirs;	/* our children *//** d_alias and d_rcu can share memory*/union {struct hlist_node d_alias;	/* inode alias list */struct rcu_head d_rcu;} d_u;
};

文件路径是通过目录树来组织的，如/usr/bin，首先根目录/对应一个dentry对象，然后bin目录对应一个dentry对象，每个目录项的名字保存在d_name成员中，这些目录通过d_child链表组织成树的结构，例如根目录下所有的dentry对象都链接在根目录中的d_child链表中，同时各个dentry对象的d_parent指向父目录的dentry对象。

不只是目录才有目录项，文件也有dentry对象。

根据路径定位一个文件过程就是一层一层找对应的dentry对象，根目录的dentry对象是在mount时建立的，从d_child找对应的dentry对象，如果内存中没有就从磁盘中读出。

各个具体文件系统对目录操作d_op也不一样，定义如下：

struct dentry_operations {int (*d_revalidate)(struct dentry *, unsigned int);int (*d_weak_revalidate)(struct dentry *, unsigned int);int (*d_hash)(const struct dentry *, struct qstr *);int (*d_compare)(const struct dentry *, const struct dentry *,unsigned int, const char *, const struct qstr *);int (*d_delete)(const struct dentry *);void (*d_release)(struct dentry *);void (*d_prune)(struct dentry *);void (*d_iput)(struct dentry *, struct inode *);char *(*d_dname)(struct dentry *, char *, int);struct vfsmount *(*d_automount)(struct path *);int (*d_manage)(struct dentry *, bool);struct inode *(*d_select_inode)(struct dentry *, unsigned);
} ____cacheline_aligned;

d_op为NULL时表示使用默认dentry操作方式，我们的ext4是这样使用默认的吗？