vfs.txt

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

struct inode_operations {
	struct file_operations * default_file_ops;
	int (*create) (struct inode *,struct dentry *,int);
	int (*lookup) (struct inode *,struct dentry *);
	int (*link) (struct dentry *,struct inode *,struct dentry *);
	int (*unlink) (struct inode *,struct dentry *);
	int (*symlink) (struct inode *,struct dentry *,const char *);
	int (*mkdir) (struct inode *,struct dentry *,int);
	int (*rmdir) (struct inode *,struct dentry *);
	int (*mknod) (struct inode *,struct dentry *,int,int);
	int (*rename) (struct inode *, struct dentry *,
			struct inode *, struct dentry *);
	int (*readlink) (struct dentry *, char *,int);
	struct dentry * (*follow_link) (struct dentry *, struct dentry *);
	int (*readpage) (struct file *, struct page *);
	int (*writepage) (struct file *, struct page *);
	int (*bmap) (struct inode *,int);
	void (*truncate) (struct inode *);
	int (*permission) (struct inode *, int);
	int (*smap) (struct inode *,int);
	int (*updatepage) (struct file *, struct page *, const char *,
				unsigned long, unsigned int, int);
	int (*revalidate) (struct dentry *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

  default_file_ops: this is a pointer to a "struct file_operations"
	which describes how to open and then manipulate open files

  create: called by the open(2) and creat(2) system calls. Only
	required if you want to support regular files. The dentry you
	get should not have an inode (i.e. it should be a negative
	dentry). Here you will probably call d_instantiate() with the
	dentry and the newly created inode

  lookup: called when the VFS needs to lookup an inode in a parent
	directory. The name to look for is found in the dentry. This
	method must call d_add() to insert the found inode into the
	dentry. The "i_count" field in the inode structure should be
	incremented. If the named inode does not exist a NULL inode
	should be inserted into the dentry (this is called a negative
	dentry). Returning an error code from this routine must only
	be done on a real error, otherwise creating inodes with system
	calls like create(2), mknod(2), mkdir(2) and so on will fail.
	If you wish to overload the dentry methods then you should
	initialise the "d_dop" field in the dentry; this is a pointer
	to a struct "dentry_operations".
	This method is called with the directory inode semaphore held

  link: called by the link(2) system call. Only required if you want
	to support hard links. You will probably need to call
	d_instantiate() just as you would in the create() method

  unlink: called by the unlink(2) system call. Only required if you
	want to support deleting inodes

  symlink: called by the symlink(2) system call. Only required if you
	want to support symlinks. You will probably need to call
	d_instantiate() just as you would in the create() method

  mkdir: called by the mkdir(2) system call. Only required if you want
	to support creating subdirectories. You will probably need to
	call d_instantiate() just as you would in the create() method

  rmdir: called by the rmdir(2) system call. Only required if you want
	to support deleting subdirectories

  mknod: called by the mknod(2) system call to create a device (char,
	block) inode or a named pipe (FIFO) or socket. Only required
	if you want to support creating these types of inodes. You
	will probably need to call d_instantiate() just as you would
	in the create() method

  readlink: called by the readlink(2) system call. Only required if
	you want to support reading symbolic links

  follow_link: called by the VFS to follow a symbolic link to the
	inode it points to. Only required if you want to support
	symbolic links


struct file_operations                                                <section>
======================

This describes how the VFS can manipulate an open file. As of kernel
2.1.99, the following members are defined:

struct file_operations {
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, struct dentry *);
	int (*fasync) (struct file *, int);
	int (*check_media_change) (kdev_t dev);
	int (*revalidate) (kdev_t dev);
	int (*lock) (struct file *, int, struct file_lock *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

  llseek: called when the VFS needs to move the file position index

  read: called by read(2) and related system calls

  write: called by write(2) and related system calls

  readdir: called when the VFS needs to read the directory contents

  poll: called by the VFS when a process wants to check if there is
	activity on this file and (optionally) go to sleep until there
	is activity. Called by the select(2) and poll(2) system calls

  ioctl: called by the ioctl(2) system call

  mmap: called by the mmap(2) system call

  open: called by the VFS when an inode should be opened. When the VFS
	opens a file, it creates a new "struct file" and initialises
	the "f_op" file operations member with the "default_file_ops"
	field in the inode structure. It then calls the open method
	for the newly allocated file structure. You might think that
	the open method really belongs in "struct inode_operations",
	and you may be right. I think it's done the way it is because
	it makes filesystems simpler to implement. The open() method
	is a good place to initialise the "private_data" member in the
	file structure if you want to point to a device structure

  release: called when the last reference to an open file is closed

  fsync: called by the fsync(2) system call

  fasync: called by the fcntl(2) system call when asynchronous
	(non-blocking) mode is enabled for a file

Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node
(character or block special) most filesystems will call special
support routines in the VFS which will locate the required device
driver information. These support routines replace the filesystem file
operations with those for the device driver, and then proceed to call
the new open() method for the file. This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
method. Note the devfs (the Device FileSystem) has a more direct path
from device node to device driver (this is an unofficial kernel
patch).


struct dentry_operations                                              <section>
========================

This describes how a filesystem can overload the standard dentry
operations. Dentries and the dcache are the domain of the VFS and the
individual filesystem implementations. Device drivers have no business
here. These methods may be set to NULL, as they are either optional or
the VFS uses a default. As of kernel 2.1.99, the following members are
defined:

struct dentry_operations {
	int (*d_revalidate)(struct dentry *);
	int (*d_hash) (struct dentry *, struct qstr *);
	int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
	void (*d_delete)(struct dentry *);
	void (*d_release)(struct dentry *);
	void (*d_iput)(struct dentry *, struct inode *);
};

  d_revalidate: called when the VFS needs to revalidate a dentry. This
	is called whenever a name lookup finds a dentry in the
	dcache. Most filesystems leave this as NULL, because all their
	dentries in the dcache are valid

  d_hash: called when the VFS adds a dentry to the hash table

  d_compare: called when a dentry should be compared with another

  d_delete: called when the last reference to a dentry is
	deleted. This means no-one is using the dentry, however it is
	still valid and in the dcache

  d_release: called when a dentry is really deallocated

  d_iput: called when a dentry looses its inode (just prior to its
	being deallocated). The default when this is NULL is that the
	VFS calls iput(). If you define this method, you must call
	iput() yourself

Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.

There are a number of functions defined which permit a filesystem to
manipulate dentries:

  dget: open a new handle for an existing dentry (this just increments
	the usage count)

  dput: close a handle for a dentry (decrements the usage count). If
	the usage count drops to 0, the "d_delete" method is called
	and the dentry is placed on the unused list if the dentry is
	still in its parents hash list. Putting the dentry on the
	unused list just means that if the system needs some RAM, it
	goes through the unused list of dentries and deallocates them.
	If the dentry has already been unhashed and the usage count
	drops to 0, in this case the dentry is deallocated after the
	"d_delete" method is called

  d_drop: this unhashes a dentry from its parents hash list. A
	subsequent call to dput() will dellocate the dentry if its
	usage count drops to 0

  d_delete: delete a dentry. If there are no other open references to
	the dentry then the dentry is turned into a negative dentry
	(the d_iput() method is called). If there are other
	references, then d_drop() is called instead

  d_add: add a dentry to its parents hash list and then calls
	d_instantiate()

  d_instantiate: add a dentry to the alias hash list for the inode and
	updates the "d_inode" member. The "i_count" member in the
	inode structure should be set/incremented. If the inode
	pointer is NULL, the dentry is called a "negative
	dentry". This function is commonly called when an inode is
	created for an existing negative dentry