932 lines
38 KiB
Plaintext
932 lines
38 KiB
Plaintext
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Overview of the Linux Virtual File System
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Original author: Richard Gooch <rgooch@atnf.csiro.au>
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Last updated on October 28, 2005
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Copyright (C) 1999 Richard Gooch
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Copyright (C) 2005 Pekka Enberg
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This file is released under the GPLv2.
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Introduction
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============
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The Virtual File System (also known as the Virtual Filesystem Switch)
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is the software layer in the kernel that provides the filesystem
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interface to userspace programs. It also provides an abstraction
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within the kernel which allows different filesystem implementations to
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coexist.
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VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
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on are called from a process context. Filesystem locking is described
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in the document Documentation/filesystems/Locking.
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Directory Entry Cache (dcache)
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------------------------------
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The VFS implements the open(2), stat(2), chmod(2), and similar system
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calls. The pathname argument that is passed to them is used by the VFS
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to search through the directory entry cache (also known as the dentry
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cache or dcache). This provides a very fast look-up mechanism to
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translate a pathname (filename) into a specific dentry. Dentries live
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in RAM and are never saved to disc: they exist only for performance.
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The dentry cache is meant to be a view into your entire filespace. As
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most computers cannot fit all dentries in the RAM at the same time,
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some bits of the cache are missing. In order to resolve your pathname
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into a dentry, the VFS may have to resort to creating dentries along
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the way, and then loading the inode. This is done by looking up the
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inode.
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The Inode Object
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----------------
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An individual dentry usually has a pointer to an inode. Inodes are
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filesystem objects such as regular files, directories, FIFOs and other
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beasts. They live either on the disc (for block device filesystems)
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or in the memory (for pseudo filesystems). Inodes that live on the
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disc are copied into the memory when required and changes to the inode
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are written back to disc. A single inode can be pointed to by multiple
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dentries (hard links, for example, do this).
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To look up an inode requires that the VFS calls the lookup() method of
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the parent directory inode. This method is installed by the specific
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filesystem implementation that the inode lives in. Once the VFS has
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the required dentry (and hence the inode), we can do all those boring
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things like open(2) the file, or stat(2) it to peek at the inode
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data. The stat(2) operation is fairly simple: once the VFS has the
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dentry, it peeks at the inode data and passes some of it back to
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userspace.
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The File Object
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---------------
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Opening a file requires another operation: allocation of a file
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structure (this is the kernel-side implementation of file
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descriptors). The freshly allocated file structure is initialized with
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a pointer to the dentry and a set of file operation member functions.
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These are taken from the inode data. The open() file method is then
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called so the specific filesystem implementation can do it's work. You
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can see that this is another switch performed by the VFS. The file
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structure is placed into the file descriptor table for the process.
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Reading, writing and closing files (and other assorted VFS operations)
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is done by using the userspace file descriptor to grab the appropriate
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file structure, and then calling the required file structure method to
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do whatever is required. For as long as the file is open, it keeps the
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dentry in use, which in turn means that the VFS inode is still in use.
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Registering and Mounting a Filesystem
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=====================================
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To register and unregister a filesystem, use the following API
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functions:
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#include <linux/fs.h>
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extern int register_filesystem(struct file_system_type *);
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extern int unregister_filesystem(struct file_system_type *);
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The passed struct file_system_type describes your filesystem. When a
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request is made to mount a device onto a directory in your filespace,
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the VFS will call the appropriate get_sb() method for the specific
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filesystem. The dentry for the mount point will then be updated to
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point to the root inode for the new filesystem.
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You can see all filesystems that are registered to the kernel in the
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file /proc/filesystems.
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struct file_system_type
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-----------------------
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This describes the filesystem. As of kernel 2.6.13, the following
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members are defined:
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struct file_system_type {
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const char *name;
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int fs_flags;
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int (*get_sb) (struct file_system_type *, int,
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const char *, void *, struct vfsmount *);
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void (*kill_sb) (struct super_block *);
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struct module *owner;
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struct file_system_type * next;
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struct list_head fs_supers;
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};
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name: the name of the filesystem type, such as "ext2", "iso9660",
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"msdos" and so on
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fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
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get_sb: the method to call when a new instance of this
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filesystem should be mounted
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kill_sb: the method to call when an instance of this filesystem
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should be unmounted
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owner: for internal VFS use: you should initialize this to THIS_MODULE in
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most cases.
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next: for internal VFS use: you should initialize this to NULL
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The get_sb() method has the following arguments:
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struct super_block *sb: the superblock structure. This is partially
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initialized by the VFS and the rest must be initialized by the
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get_sb() method
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int flags: mount flags
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const char *dev_name: the device name we are mounting.
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void *data: arbitrary mount options, usually comes as an ASCII
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string
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int silent: whether or not to be silent on error
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The get_sb() method must determine if the block device specified
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in the superblock contains a filesystem of the type the method
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supports. On success the method returns the superblock pointer, on
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failure it returns NULL.
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The most interesting member of the superblock structure that the
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get_sb() method fills in is the "s_op" field. This is a pointer to
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a "struct super_operations" which describes the next level of the
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filesystem implementation.
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Usually, a filesystem uses one of the generic get_sb() implementations
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and provides a fill_super() method instead. The generic methods are:
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get_sb_bdev: mount a filesystem residing on a block device
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get_sb_nodev: mount a filesystem that is not backed by a device
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get_sb_single: mount a filesystem which shares the instance between
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all mounts
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A fill_super() method implementation has the following arguments:
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struct super_block *sb: the superblock structure. The method fill_super()
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must initialize this properly.
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void *data: arbitrary mount options, usually comes as an ASCII
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string
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int silent: whether or not to be silent on error
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The Superblock Object
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=====================
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A superblock object represents a mounted filesystem.
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struct super_operations
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-----------------------
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This describes how the VFS can manipulate the superblock of your
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filesystem. As of kernel 2.6.13, the following members are defined:
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struct super_operations {
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struct inode *(*alloc_inode)(struct super_block *sb);
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void (*destroy_inode)(struct inode *);
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void (*read_inode) (struct inode *);
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void (*dirty_inode) (struct inode *);
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int (*write_inode) (struct inode *, int);
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void (*put_inode) (struct inode *);
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void (*drop_inode) (struct inode *);
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void (*delete_inode) (struct inode *);
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void (*put_super) (struct super_block *);
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void (*write_super) (struct super_block *);
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int (*sync_fs)(struct super_block *sb, int wait);
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void (*write_super_lockfs) (struct super_block *);
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void (*unlockfs) (struct super_block *);
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int (*statfs) (struct dentry *, struct kstatfs *);
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int (*remount_fs) (struct super_block *, int *, char *);
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void (*clear_inode) (struct inode *);
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void (*umount_begin) (struct super_block *);
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void (*sync_inodes) (struct super_block *sb,
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struct writeback_control *wbc);
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int (*show_options)(struct seq_file *, struct vfsmount *);
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ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
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ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
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};
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All methods are called without any locks being held, unless otherwise
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noted. This means that most methods can block safely. All methods are
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only called from a process context (i.e. not from an interrupt handler
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or bottom half).
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alloc_inode: this method is called by inode_alloc() to allocate memory
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for struct inode and initialize it. If this function is not
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defined, a simple 'struct inode' is allocated. Normally
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alloc_inode will be used to allocate a larger structure which
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contains a 'struct inode' embedded within it.
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destroy_inode: this method is called by destroy_inode() to release
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resources allocated for struct inode. It is only required if
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->alloc_inode was defined and simply undoes anything done by
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->alloc_inode.
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read_inode: this method is called to read a specific inode from the
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mounted filesystem. The i_ino member in the struct inode is
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initialized by the VFS to indicate which inode to read. Other
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members are filled in by this method.
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You can set this to NULL and use iget5_locked() instead of iget()
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to read inodes. This is necessary for filesystems for which the
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inode number is not sufficient to identify an inode.
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dirty_inode: this method is called by the VFS to mark an inode dirty.
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write_inode: this method is called when the VFS needs to write an
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inode to disc. The second parameter indicates whether the write
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should be synchronous or not, not all filesystems check this flag.
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put_inode: called when the VFS inode is removed from the inode
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cache.
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drop_inode: called when the last access to the inode is dropped,
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with the inode_lock spinlock held.
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This method should be either NULL (normal UNIX filesystem
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semantics) or "generic_delete_inode" (for filesystems that do not
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want to cache inodes - causing "delete_inode" to always be
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called regardless of the value of i_nlink)
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The "generic_delete_inode()" behavior is equivalent to the
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old practice of using "force_delete" in the put_inode() case,
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but does not have the races that the "force_delete()" approach
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had.
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delete_inode: called when the VFS wants to delete an inode
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put_super: called when the VFS wishes to free the superblock
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(i.e. unmount). This is called with the superblock lock held
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write_super: called when the VFS superblock needs to be written to
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disc. This method is optional
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sync_fs: called when VFS is writing out all dirty data associated with
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a superblock. The second parameter indicates whether the method
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should wait until the write out has been completed. Optional.
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write_super_lockfs: called when VFS is locking a filesystem and
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forcing it into a consistent state. This method is currently
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used by the Logical Volume Manager (LVM).
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unlockfs: called when VFS is unlocking a filesystem and making it writable
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again.
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statfs: called when the VFS needs to get filesystem statistics. This
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is called with the kernel lock held
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remount_fs: called when the filesystem is remounted. This is called
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with the kernel lock held
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clear_inode: called then the VFS clears the inode. Optional
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umount_begin: called when the VFS is unmounting a filesystem.
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sync_inodes: called when the VFS is writing out dirty data associated with
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a superblock.
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show_options: called by the VFS to show mount options for /proc/<pid>/mounts.
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quota_read: called by the VFS to read from filesystem quota file.
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quota_write: called by the VFS to write to filesystem quota file.
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The read_inode() method is responsible for filling in the "i_op"
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field. This is a pointer to a "struct inode_operations" which
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describes the methods that can be performed on individual inodes.
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The Inode Object
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================
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An inode object represents an object within the filesystem.
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struct inode_operations
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-----------------------
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This describes how the VFS can manipulate an inode in your
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filesystem. As of kernel 2.6.13, the following members are defined:
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struct inode_operations {
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int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
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struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
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int (*link) (struct dentry *,struct inode *,struct dentry *);
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int (*unlink) (struct inode *,struct dentry *);
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int (*symlink) (struct inode *,struct dentry *,const char *);
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int (*mkdir) (struct inode *,struct dentry *,int);
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int (*rmdir) (struct inode *,struct dentry *);
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int (*mknod) (struct inode *,struct dentry *,int,dev_t);
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int (*rename) (struct inode *, struct dentry *,
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struct inode *, struct dentry *);
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int (*readlink) (struct dentry *, char __user *,int);
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void * (*follow_link) (struct dentry *, struct nameidata *);
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void (*put_link) (struct dentry *, struct nameidata *, void *);
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void (*truncate) (struct inode *);
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int (*permission) (struct inode *, int, struct nameidata *);
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int (*setattr) (struct dentry *, struct iattr *);
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int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
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int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
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ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
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ssize_t (*listxattr) (struct dentry *, char *, size_t);
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int (*removexattr) (struct dentry *, const char *);
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};
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Again, all methods are called without any locks being held, unless
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otherwise noted.
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create: called by the open(2) and creat(2) system calls. Only
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required if you want to support regular files. The dentry you
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get should not have an inode (i.e. it should be a negative
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dentry). Here you will probably call d_instantiate() with the
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dentry and the newly created inode
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lookup: called when the VFS needs to look up an inode in a parent
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directory. The name to look for is found in the dentry. This
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method must call d_add() to insert the found inode into the
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dentry. The "i_count" field in the inode structure should be
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incremented. If the named inode does not exist a NULL inode
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should be inserted into the dentry (this is called a negative
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dentry). Returning an error code from this routine must only
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be done on a real error, otherwise creating inodes with system
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calls like create(2), mknod(2), mkdir(2) and so on will fail.
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If you wish to overload the dentry methods then you should
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initialise the "d_dop" field in the dentry; this is a pointer
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to a struct "dentry_operations".
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This method is called with the directory inode semaphore held
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link: called by the link(2) system call. Only required if you want
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to support hard links. You will probably need to call
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d_instantiate() just as you would in the create() method
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unlink: called by the unlink(2) system call. Only required if you
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want to support deleting inodes
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symlink: called by the symlink(2) system call. Only required if you
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want to support symlinks. You will probably need to call
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d_instantiate() just as you would in the create() method
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mkdir: called by the mkdir(2) system call. Only required if you want
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to support creating subdirectories. You will probably need to
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call d_instantiate() just as you would in the create() method
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rmdir: called by the rmdir(2) system call. Only required if you want
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to support deleting subdirectories
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mknod: called by the mknod(2) system call to create a device (char,
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block) inode or a named pipe (FIFO) or socket. Only required
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if you want to support creating these types of inodes. You
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will probably need to call d_instantiate() just as you would
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in the create() method
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rename: called by the rename(2) system call to rename the object to
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have the parent and name given by the second inode and dentry.
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readlink: called by the readlink(2) system call. Only required if
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you want to support reading symbolic links
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follow_link: called by the VFS to follow a symbolic link to the
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inode it points to. Only required if you want to support
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symbolic links. This method returns a void pointer cookie
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that is passed to put_link().
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put_link: called by the VFS to release resources allocated by
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follow_link(). The cookie returned by follow_link() is passed
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to this method as the last parameter. It is used by
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filesystems such as NFS where page cache is not stable
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(i.e. page that was installed when the symbolic link walk
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started might not be in the page cache at the end of the
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walk).
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truncate: called by the VFS to change the size of a file. The
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i_size field of the inode is set to the desired size by the
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VFS before this method is called. This method is called by
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the truncate(2) system call and related functionality.
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permission: called by the VFS to check for access rights on a POSIX-like
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filesystem.
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setattr: called by the VFS to set attributes for a file. This method
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is called by chmod(2) and related system calls.
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getattr: called by the VFS to get attributes of a file. This method
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is called by stat(2) and related system calls.
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setxattr: called by the VFS to set an extended attribute for a file.
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Extended attribute is a name:value pair associated with an
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inode. This method is called by setxattr(2) system call.
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getxattr: called by the VFS to retrieve the value of an extended
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attribute name. This method is called by getxattr(2) function
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call.
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listxattr: called by the VFS to list all extended attributes for a
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given file. This method is called by listxattr(2) system call.
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removexattr: called by the VFS to remove an extended attribute from
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a file. This method is called by removexattr(2) system call.
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The Address Space Object
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========================
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The address space object is used to group and manage pages in the page
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cache. It can be used to keep track of the pages in a file (or
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anything else) and also track the mapping of sections of the file into
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process address spaces.
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There are a number of distinct yet related services that an
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address-space can provide. These include communicating memory
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pressure, page lookup by address, and keeping track of pages tagged as
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Dirty or Writeback.
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The first can be used independently to the others. The VM can try to
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either write dirty pages in order to clean them, or release clean
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pages in order to reuse them. To do this it can call the ->writepage
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method on dirty pages, and ->releasepage on clean pages with
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PagePrivate set. Clean pages without PagePrivate and with no external
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references will be released without notice being given to the
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address_space.
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To achieve this functionality, pages need to be placed on an LRU with
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lru_cache_add and mark_page_active needs to be called whenever the
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page is used.
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Pages are normally kept in a radix tree index by ->index. This tree
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maintains information about the PG_Dirty and PG_Writeback status of
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each page, so that pages with either of these flags can be found
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quickly.
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The Dirty tag is primarily used by mpage_writepages - the default
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->writepages method. It uses the tag to find dirty pages to call
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->writepage on. If mpage_writepages is not used (i.e. the address
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provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
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almost unused. write_inode_now and sync_inode do use it (through
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__sync_single_inode) to check if ->writepages has been successful in
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writing out the whole address_space.
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The Writeback tag is used by filemap*wait* and sync_page* functions,
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via wait_on_page_writeback_range, to wait for all writeback to
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complete. While waiting ->sync_page (if defined) will be called on
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each page that is found to require writeback.
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|
|
An address_space handler may attach extra information to a page,
|
|
typically using the 'private' field in the 'struct page'. If such
|
|
information is attached, the PG_Private flag should be set. This will
|
|
cause various VM routines to make extra calls into the address_space
|
|
handler to deal with that data.
|
|
|
|
An address space acts as an intermediate between storage and
|
|
application. Data is read into the address space a whole page at a
|
|
time, and provided to the application either by copying of the page,
|
|
or by memory-mapping the page.
|
|
Data is written into the address space by the application, and then
|
|
written-back to storage typically in whole pages, however the
|
|
address_space has finer control of write sizes.
|
|
|
|
The read process essentially only requires 'readpage'. The write
|
|
process is more complicated and uses prepare_write/commit_write or
|
|
set_page_dirty to write data into the address_space, and writepage,
|
|
sync_page, and writepages to writeback data to storage.
|
|
|
|
Adding and removing pages to/from an address_space is protected by the
|
|
inode's i_mutex.
|
|
|
|
When data is written to a page, the PG_Dirty flag should be set. It
|
|
typically remains set until writepage asks for it to be written. This
|
|
should clear PG_Dirty and set PG_Writeback. It can be actually
|
|
written at any point after PG_Dirty is clear. Once it is known to be
|
|
safe, PG_Writeback is cleared.
|
|
|
|
Writeback makes use of a writeback_control structure...
|
|
|
|
struct address_space_operations
|
|
-------------------------------
|
|
|
|
This describes how the VFS can manipulate mapping of a file to page cache in
|
|
your filesystem. As of kernel 2.6.16, the following members are defined:
|
|
|
|
struct address_space_operations {
|
|
int (*writepage)(struct page *page, struct writeback_control *wbc);
|
|
int (*readpage)(struct file *, struct page *);
|
|
int (*sync_page)(struct page *);
|
|
int (*writepages)(struct address_space *, struct writeback_control *);
|
|
int (*set_page_dirty)(struct page *page);
|
|
int (*readpages)(struct file *filp, struct address_space *mapping,
|
|
struct list_head *pages, unsigned nr_pages);
|
|
int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
|
|
int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
|
|
sector_t (*bmap)(struct address_space *, sector_t);
|
|
int (*invalidatepage) (struct page *, unsigned long);
|
|
int (*releasepage) (struct page *, int);
|
|
ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
|
|
loff_t offset, unsigned long nr_segs);
|
|
struct page* (*get_xip_page)(struct address_space *, sector_t,
|
|
int);
|
|
/* migrate the contents of a page to the specified target */
|
|
int (*migratepage) (struct page *, struct page *);
|
|
};
|
|
|
|
writepage: called by the VM to write a dirty page to backing store.
|
|
This may happen for data integrity reasons (i.e. 'sync'), or
|
|
to free up memory (flush). The difference can be seen in
|
|
wbc->sync_mode.
|
|
The PG_Dirty flag has been cleared and PageLocked is true.
|
|
writepage should start writeout, should set PG_Writeback,
|
|
and should make sure the page is unlocked, either synchronously
|
|
or asynchronously when the write operation completes.
|
|
|
|
If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
|
|
try too hard if there are problems, and may choose to write out
|
|
other pages from the mapping if that is easier (e.g. due to
|
|
internal dependencies). If it chooses not to start writeout, it
|
|
should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
|
|
calling ->writepage on that page.
|
|
|
|
See the file "Locking" for more details.
|
|
|
|
readpage: called by the VM to read a page from backing store.
|
|
The page will be Locked when readpage is called, and should be
|
|
unlocked and marked uptodate once the read completes.
|
|
If ->readpage discovers that it needs to unlock the page for
|
|
some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
|
|
In this case, the page will be relocated, relocked and if
|
|
that all succeeds, ->readpage will be called again.
|
|
|
|
sync_page: called by the VM to notify the backing store to perform all
|
|
queued I/O operations for a page. I/O operations for other pages
|
|
associated with this address_space object may also be performed.
|
|
|
|
This function is optional and is called only for pages with
|
|
PG_Writeback set while waiting for the writeback to complete.
|
|
|
|
writepages: called by the VM to write out pages associated with the
|
|
address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
|
|
the writeback_control will specify a range of pages that must be
|
|
written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
|
|
and that many pages should be written if possible.
|
|
If no ->writepages is given, then mpage_writepages is used
|
|
instead. This will choose pages from the address space that are
|
|
tagged as DIRTY and will pass them to ->writepage.
|
|
|
|
set_page_dirty: called by the VM to set a page dirty.
|
|
This is particularly needed if an address space attaches
|
|
private data to a page, and that data needs to be updated when
|
|
a page is dirtied. This is called, for example, when a memory
|
|
mapped page gets modified.
|
|
If defined, it should set the PageDirty flag, and the
|
|
PAGECACHE_TAG_DIRTY tag in the radix tree.
|
|
|
|
readpages: called by the VM to read pages associated with the address_space
|
|
object. This is essentially just a vector version of
|
|
readpage. Instead of just one page, several pages are
|
|
requested.
|
|
readpages is only used for read-ahead, so read errors are
|
|
ignored. If anything goes wrong, feel free to give up.
|
|
|
|
prepare_write: called by the generic write path in VM to set up a write
|
|
request for a page. This indicates to the address space that
|
|
the given range of bytes is about to be written. The
|
|
address_space should check that the write will be able to
|
|
complete, by allocating space if necessary and doing any other
|
|
internal housekeeping. If the write will update parts of
|
|
any basic-blocks on storage, then those blocks should be
|
|
pre-read (if they haven't been read already) so that the
|
|
updated blocks can be written out properly.
|
|
The page will be locked. If prepare_write wants to unlock the
|
|
page it, like readpage, may do so and return
|
|
AOP_TRUNCATED_PAGE.
|
|
In this case the prepare_write will be retried one the lock is
|
|
regained.
|
|
|
|
Note: the page _must not_ be marked uptodate in this function
|
|
(or anywhere else) unless it actually is uptodate right now. As
|
|
soon as a page is marked uptodate, it is possible for a concurrent
|
|
read(2) to copy it to userspace.
|
|
|
|
commit_write: If prepare_write succeeds, new data will be copied
|
|
into the page and then commit_write will be called. It will
|
|
typically update the size of the file (if appropriate) and
|
|
mark the inode as dirty, and do any other related housekeeping
|
|
operations. It should avoid returning an error if possible -
|
|
errors should have been handled by prepare_write.
|
|
|
|
bmap: called by the VFS to map a logical block offset within object to
|
|
physical block number. This method is used by the FIBMAP
|
|
ioctl and for working with swap-files. To be able to swap to
|
|
a file, the file must have a stable mapping to a block
|
|
device. The swap system does not go through the filesystem
|
|
but instead uses bmap to find out where the blocks in the file
|
|
are and uses those addresses directly.
|
|
|
|
|
|
invalidatepage: If a page has PagePrivate set, then invalidatepage
|
|
will be called when part or all of the page is to be removed
|
|
from the address space. This generally corresponds to either a
|
|
truncation or a complete invalidation of the address space
|
|
(in the latter case 'offset' will always be 0).
|
|
Any private data associated with the page should be updated
|
|
to reflect this truncation. If offset is 0, then
|
|
the private data should be released, because the page
|
|
must be able to be completely discarded. This may be done by
|
|
calling the ->releasepage function, but in this case the
|
|
release MUST succeed.
|
|
|
|
releasepage: releasepage is called on PagePrivate pages to indicate
|
|
that the page should be freed if possible. ->releasepage
|
|
should remove any private data from the page and clear the
|
|
PagePrivate flag. It may also remove the page from the
|
|
address_space. If this fails for some reason, it may indicate
|
|
failure with a 0 return value.
|
|
This is used in two distinct though related cases. The first
|
|
is when the VM finds a clean page with no active users and
|
|
wants to make it a free page. If ->releasepage succeeds, the
|
|
page will be removed from the address_space and become free.
|
|
|
|
The second case if when a request has been made to invalidate
|
|
some or all pages in an address_space. This can happen
|
|
through the fadvice(POSIX_FADV_DONTNEED) system call or by the
|
|
filesystem explicitly requesting it as nfs and 9fs do (when
|
|
they believe the cache may be out of date with storage) by
|
|
calling invalidate_inode_pages2().
|
|
If the filesystem makes such a call, and needs to be certain
|
|
that all pages are invalidated, then its releasepage will
|
|
need to ensure this. Possibly it can clear the PageUptodate
|
|
bit if it cannot free private data yet.
|
|
|
|
direct_IO: called by the generic read/write routines to perform
|
|
direct_IO - that is IO requests which bypass the page cache
|
|
and transfer data directly between the storage and the
|
|
application's address space.
|
|
|
|
get_xip_page: called by the VM to translate a block number to a page.
|
|
The page is valid until the corresponding filesystem is unmounted.
|
|
Filesystems that want to use execute-in-place (XIP) need to implement
|
|
it. An example implementation can be found in fs/ext2/xip.c.
|
|
|
|
migrate_page: This is used to compact the physical memory usage.
|
|
If the VM wants to relocate a page (maybe off a memory card
|
|
that is signalling imminent failure) it will pass a new page
|
|
and an old page to this function. migrate_page should
|
|
transfer any private data across and update any references
|
|
that it has to the page.
|
|
|
|
The File Object
|
|
===============
|
|
|
|
A file object represents a file opened by a process.
|
|
|
|
|
|
struct file_operations
|
|
----------------------
|
|
|
|
This describes how the VFS can manipulate an open file. As of kernel
|
|
2.6.17, the following members are defined:
|
|
|
|
struct file_operations {
|
|
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 (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
|
|
ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, 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);
|
|
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 (*open) (struct inode *, struct file *);
|
|
int (*flush) (struct file *);
|
|
int (*release) (struct inode *, struct file *);
|
|
int (*fsync) (struct file *, struct dentry *, 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 (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
|
|
ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
|
|
ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
|
|
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 (*dir_notify)(struct file *filp, unsigned long arg);
|
|
int (*flock) (struct file *, int, struct file_lock *);
|
|
ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned
|
|
int);
|
|
ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned
|
|
int);
|
|
};
|
|
|
|
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
|
|
|
|
aio_read: called by io_submit(2) and other asynchronous I/O operations
|
|
|
|
write: called by write(2) and related system calls
|
|
|
|
aio_write: called by io_submit(2) and other asynchronous I/O operations
|
|
|
|
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
|
|
|
|
unlocked_ioctl: called by the ioctl(2) system call. Filesystems that do not
|
|
require the BKL should use this method instead of the ioctl() above.
|
|
|
|
compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
|
|
are used on 64 bit kernels.
|
|
|
|
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". 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 initialize the
|
|
"private_data" member in the file structure if you want to point
|
|
to a device structure
|
|
|
|
flush: called by the close(2) system call to flush a file
|
|
|
|
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
|
|
|
|
lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
|
|
commands
|
|
|
|
readv: called by the readv(2) system call
|
|
|
|
writev: called by the writev(2) system call
|
|
|
|
sendfile: called by the sendfile(2) system call
|
|
|
|
get_unmapped_area: called by the mmap(2) system call
|
|
|
|
check_flags: called by the fcntl(2) system call for F_SETFL command
|
|
|
|
dir_notify: called by the fcntl(2) system call for F_NOTIFY command
|
|
|
|
flock: called by the flock(2) system call
|
|
|
|
splice_write: called by the VFS to splice data from a pipe to a file. This
|
|
method is used by the splice(2) system call
|
|
|
|
splice_read: called by the VFS to splice data from file to a pipe. This
|
|
method is used by the splice(2) system call
|
|
|
|
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.
|
|
|
|
|
|
Directory Entry Cache (dcache)
|
|
==============================
|
|
|
|
|
|
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 2.6.13, the following members are
|
|
defined:
|
|
|
|
struct dentry_operations {
|
|
int (*d_revalidate)(struct dentry *, struct nameidata *);
|
|
int (*d_hash) (struct dentry *, struct qstr *);
|
|
int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
|
|
int (*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 look-up 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 loses 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.
|
|
|
|
|
|
Directory Entry Cache API
|
|
--------------------------
|
|
|
|
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 deallocate 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
|
|
|
|
d_lookup: look up a dentry given its parent and path name component
|
|
It looks up the child of that given name from the dcache
|
|
hash table. If it is found, the reference count is incremented
|
|
and the dentry is returned. The caller must use d_put()
|
|
to free the dentry when it finishes using it.
|
|
|
|
For further information on dentry locking, please refer to the document
|
|
Documentation/filesystems/dentry-locking.txt.
|
|
|
|
|
|
Resources
|
|
=========
|
|
|
|
(Note some of these resources are not up-to-date with the latest kernel
|
|
version.)
|
|
|
|
Creating Linux virtual filesystems. 2002
|
|
<http://lwn.net/Articles/13325/>
|
|
|
|
The Linux Virtual File-system Layer by Neil Brown. 1999
|
|
<http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
|
|
|
|
A tour of the Linux VFS by Michael K. Johnson. 1996
|
|
<http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
|
|
|
|
A small trail through the Linux kernel by Andries Brouwer. 2001
|
|
<http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
|