/* -*- auto-fill -*- */ Overview of the Virtual File System Richard Gooch 5-JUL-1999 Conventions used in this document
================================= Each section in this document will have the string "
" at the right-hand side of the section title. Each subsection will have "" at the right-hand side. These strings are meant to make it easier to search through the document. NOTE that the master copy of this document is available online at: http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt What is it?
=========== The Virtual File System (otherwise known as the Virtual Filesystem Switch) is the software layer in the kernel that provides the filesystem interface to userspace programmes. It also provides an abstraction within the kernel which allows different filesystem implementations to co-exist. A Quick Look At How It Works
============================ In this section I'll briefly describe how things work, before launching into the details. I'll start with describing what happens when user programmes open and manipulate files, and then look from the other view which is how a filesystem is supported and subsequently mounted. Opening a File -------------- The VFS implements the open(2), stat(2), chmod(2) and similar system calls. The pathname argument is used by the VFS to search through the directory entry cache (dentry cache or "dcache"). This provides a very fast lookup mechanism to translate a pathname (filename) into a specific dentry. An individual dentry usually has a pointer to an inode. Inodes are the things that live on disc drives, and can be regular files (you know: those things that you write data into), directories, FIFOs and other beasts. Dentries live in RAM and are never saved to disc: they exist only for performance. Inodes live on disc and are copied into memory when required. Later any changes are written back to disc. The inode that lives in RAM is a VFS inode, and it is this which the dentry points to. A single inode can be pointed to by multiple dentries (think about hardlinks). The dcache is meant to be a view into your entire filespace. Unlike Linus, most of us losers can't fit enough dentries into RAM to cover all of our filespace, so the dcache has bits missing. In order to resolve your pathname into a dentry, the VFS may have to resort to creating dentries along the way, and then loading the inode. This is done by looking up the inode. To lookup an inode (usually read from disc) requires that the VFS calls the lookup() method of the parent directory inode. This method is installed by the specific filesystem implementation that the inode lives in. There will be more on this later. Once the VFS has the required dentry (and hence the inode), we can do all those boring things like open(2) the file, or stat(2) it to peek at the inode data. The stat(2) operation is fairly simple: once the VFS has the dentry, it peeks at the inode data and passes some of it back to userspace. Opening a file requires another operation: allocation of a file structure (this is the kernel-side implementation of file descriptors). The freshly allocated file structure is initialised with a pointer to the dentry and a set of file operation member functions. These are taken from the inode data. The open() file method is then called so the specific filesystem implementation can do it's work. You can see that this is another switch performed by the VFS. The file structure is placed into the file descriptor table for the process. Reading, writing and closing files (and other assorted VFS operations) is done by using the userspace file descriptor to grab the appropriate file structure, and then calling the required file structure method function to do whatever is required. For as long as the file is open, it keeps the dentry "open" (in use), which in turn means that the VFS inode is still in use. All VFS system calls (i.e. open(2), stat(2), read(2), write(2), chmod(2) and so on) are called from a process context. You should assume that these calls are made without any kernel locks being held. This means that the processes may be executing the same piece of filesystem or driver code at the same time, on different processors. You should ensure that access to shared resources is protected by appropriate locks. Registering and Mounting a Filesystem ------------------------------------- If you want to support a new kind of filesystem in the kernel, all you need to do is call register_filesystem(). You pass a structure describing the filesystem implementation (struct file_system_type) which is then added to an internal table of supported filesystems. You can do: % cat /proc/filesystems to see what filesystems are currently available on your system. When a request is made to mount a block device onto a directory in your filespace the VFS will call the appropriate method for the specific filesystem. The dentry for the mount point will then be updated to point to the root inode for the new filesystem. It's now time to look at things in more detail. struct file_system_type
======================= This describes the filesystem. As of kernel 2.1.99, the following members are defined: struct file_system_type { const char *name; int fs_flags; struct super_block *(*read_super) (struct super_block *, void *, int); struct file_system_type * next; }; name: the name of the filesystem type, such as "ext2", "iso9660", "msdos" and so on fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) read_super: the method to call when a new instance of this filesystem should be mounted next: for internal VFS use: you should initialise this to NULL The read_super() method has the following arguments: struct super_block *sb: the superblock structure. This is partially initialised by the VFS and the rest must be initialised by the read_super() method void *data: arbitrary mount options, usually comes as an ASCII string int silent: whether or not to be silent on error The read_super() method must determine if the block device specified in the superblock contains a filesystem of the type the method supports. On success the method returns the superblock pointer, on failure it returns NULL. The most interesting member of the superblock structure that the read_super() method fills in is the "s_op" field. This is a pointer to a "struct super_operations" which describes the next level of the filesystem implementation. struct super_operations
======================= This describes how the VFS can manipulate the superblock of your filesystem. As of kernel 2.1.99, the following members are defined: struct super_operations { void (*read_inode) (struct inode *); void (*write_inode) (struct inode *); void (*put_inode) (struct inode *); void (*delete_inode) (struct inode *); int (*notify_change) (struct dentry *, struct iattr *); void (*put_super) (struct super_block *); void (*write_super) (struct super_block *); int (*statfs) (struct super_block *, struct statfs *, int); int (*remount_fs) (struct super_block *, int *, char *); void (*clear_inode) (struct inode *); }; All methods are called without any locks being held, unless otherwise noted. This means that most methods can block safely. All methods are only called from a process context (i.e. not from an interrupt handler or bottom half). read_inode: this method is called to read a specific inode from the mounted filesystem. The "i_ino" member in the "struct inode" will be initialised by the VFS to indicate which inode to read. Other members are filled in by this method write_inode: this method is called when the VFS needs to write an inode to disc put_inode: called when the VFS inode is removed from the inode cache. This method is optional delete_inode: called when the VFS wants to delete an inode notify_change: called when VFS inode attributes are changed. If this is NULL the VFS falls back to the write_inode() method. This is called with the kernel lock held put_super: called when the VFS wishes to free the superblock (i.e. unmount). This is called with the superblock lock held write_super: called when the VFS superblock needs to be written to disc. This method is optional statfs: called when the VFS needs to get filesystem statistics. This is called with the kernel lock held remount_fs: called when the filesystem is remounted. This is called with the kernel lock held clear_inode: called then the VFS clears the inode. Optional The read_inode() method is responsible for filling in the "i_op" field. This is a pointer to a "struct inode_operations" which describes the methods that can be performed on individual inodes. struct inode_operations
======================= This describes how the VFS can manipulate an inode in your filesystem. As of kernel 2.1.99, the following members are defined: 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
====================== 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
======================== 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