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671 lines
28 KiB
/* -*- auto-fill -*- */
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Overview of the Virtual File System
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Richard Gooch <rgooch@atnf.csiro.au>
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5-JUL-1999
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Conventions used in this document <section>
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=================================
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Each section in this document will have the string "<section>" at the
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right-hand side of the section title. Each subsection will have
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"<subsection>" at the right-hand side. These strings are meant to make
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it easier to search through the document.
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NOTE that the master copy of this document is available online at:
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http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt
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What is it? <section>
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===========
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The Virtual File System (otherwise known as the Virtual Filesystem
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Switch) is the software layer in the kernel that provides the
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filesystem interface to userspace programs. It also provides an
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abstraction within the kernel which allows different filesystem
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implementations to co-exist.
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A Quick Look At How It Works <section>
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============================
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In this section I'll briefly describe how things work, before
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launching into the details. I'll start with describing what happens
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when user programs open and manipulate files, and then look from the
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other view which is how a filesystem is supported and subsequently
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mounted.
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Opening a File <subsection>
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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 is used by the VFS to search through the
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directory entry cache (dentry cache or "dcache"). This provides a very
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fast look-up mechanism to translate a pathname (filename) into a
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specific dentry.
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An individual dentry usually has a pointer to an inode. Inodes are the
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things that live on disc drives, and can be regular files (you know:
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those things that you write data into), directories, FIFOs and other
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beasts. Dentries live in RAM and are never saved to disc: they exist
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only for performance. Inodes live on disc and are copied into memory
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when required. Later any changes are written back to disc. The inode
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that lives in RAM is a VFS inode, and it is this which the dentry
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points to. A single inode can be pointed to by multiple dentries
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(think about hardlinks).
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The dcache is meant to be a view into your entire filespace. Unlike
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Linus, most of us losers can't fit enough dentries into RAM to cover
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all of our filespace, so the dcache has bits missing. In order to
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resolve your pathname into a dentry, the VFS may have to resort to
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creating dentries along the way, and then loading the inode. This is
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done by looking up the inode.
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To look up an inode (usually read from disc) requires that the VFS
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calls the lookup() method of the parent directory inode. This method
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is installed by the specific filesystem implementation that the inode
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lives in. There will be more on this later.
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Once the VFS has the required dentry (and hence the inode), we can do
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all those boring things like open(2) the file, or stat(2) it to peek
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at the inode data. The stat(2) operation is fairly simple: once the
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VFS has the dentry, it peeks at the inode data and passes some of it
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back to userspace.
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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 initialised 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.
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The file structure is placed into the file descriptor table for the
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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
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function to do whatever is required.
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For as long as the file is open, it keeps the dentry "open" (in use),
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which in turn means that the VFS inode is still in use.
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All VFS system calls (i.e. open(2), stat(2), read(2), write(2),
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chmod(2) and so on) are called from a process context. You should
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assume that these calls are made without any kernel locks being
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held. This means that the processes may be executing the same piece of
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filesystem or driver code at the same time, on different
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processors. You should ensure that access to shared resources is
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protected by appropriate locks.
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Registering and Mounting a Filesystem <subsection>
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-------------------------------------
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If you want to support a new kind of filesystem in the kernel, all you
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need to do is call register_filesystem(). You pass a structure
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describing the filesystem implementation (struct file_system_type)
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which is then added to an internal table of supported filesystems. You
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can do:
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% cat /proc/filesystems
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to see what filesystems are currently available on your system.
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When a request is made to mount a block device onto a directory in
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your filespace the VFS will call the appropriate method for the
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specific filesystem. The dentry for the mount point will then be
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updated to point to the root inode for the new filesystem.
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It's now time to look at things in more detail.
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struct file_system_type <section>
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=======================
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This describes the filesystem. As of kernel 2.1.99, 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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struct super_block *(*read_super) (struct super_block *, void *, int);
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struct file_system_type * next;
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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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read_super: the method to call when a new instance of this
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filesystem should be mounted
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next: for internal VFS use: you should initialise this to NULL
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The read_super() method has the following arguments:
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struct super_block *sb: the superblock structure. This is partially
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initialised by the VFS and the rest must be initialised by the
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read_super() method
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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 read_super() 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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read_super() 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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struct super_operations <section>
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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 2.1.99, the following members are defined:
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struct super_operations {
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void (*read_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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int (*notify_change) (struct dentry *, struct iattr *);
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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 (*statfs) (struct super_block *, struct statfs *, int);
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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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};
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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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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"
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will be initialised by the VFS to indicate which inode to
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read. Other members are filled in by this method
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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. This method is optional
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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()" behaviour 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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notify_change: called when VFS inode attributes are changed. If this
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is NULL the VFS falls back to the write_inode() method. This
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is called with the kernel lock held
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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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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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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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struct inode_operations <section>
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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 2.1.99, the following members are defined:
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struct inode_operations {
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struct file_operations * default_file_ops;
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int (*create) (struct inode *,struct dentry *,int);
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int (*lookup) (struct inode *,struct dentry *);
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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 *,int);
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struct dentry * (*follow_link) (struct dentry *, struct dentry *);
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int (*readpage) (struct file *, struct page *);
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int (*writepage) (struct page *page, struct writeback_control *wbc);
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int (*bmap) (struct inode *,int);
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void (*truncate) (struct inode *);
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int (*permission) (struct inode *, int);
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int (*smap) (struct inode *,int);
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int (*updatepage) (struct file *, struct page *, const char *,
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unsigned long, unsigned int, int);
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int (*revalidate) (struct dentry *);
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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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default_file_ops: this is a pointer to a "struct file_operations"
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which describes how to open and then manipulate open files
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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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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
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struct file_operations <section>
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======================
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This describes how the VFS can manipulate an open file. As of kernel
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2.1.99, the following members are defined:
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struct file_operations {
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loff_t (*llseek) (struct file *, loff_t, int);
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ssize_t (*read) (struct file *, char *, size_t, loff_t *);
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ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
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int (*readdir) (struct file *, void *, filldir_t);
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unsigned int (*poll) (struct file *, struct poll_table_struct *);
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int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
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int (*mmap) (struct file *, struct vm_area_struct *);
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int (*open) (struct inode *, struct file *);
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int (*release) (struct inode *, struct file *);
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int (*fsync) (struct file *, struct dentry *);
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int (*fasync) (struct file *, int);
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int (*check_media_change) (kdev_t dev);
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int (*revalidate) (kdev_t dev);
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int (*lock) (struct file *, int, struct file_lock *);
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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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llseek: called when the VFS needs to move the file position index
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read: called by read(2) and related system calls
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write: called by write(2) and related system calls
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readdir: called when the VFS needs to read the directory contents
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poll: called by the VFS when a process wants to check if there is
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activity on this file and (optionally) go to sleep until there
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is activity. Called by the select(2) and poll(2) system calls
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ioctl: called by the ioctl(2) system call
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mmap: called by the mmap(2) system call
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open: called by the VFS when an inode should be opened. When the VFS
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opens a file, it creates a new "struct file" and initialises
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the "f_op" file operations member with the "default_file_ops"
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field in the inode structure. It then calls the open method
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for the newly allocated file structure. You might think that
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the open method really belongs in "struct inode_operations",
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and you may be right. I think it's done the way it is because
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it makes filesystems simpler to implement. The open() method
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is a good place to initialise the "private_data" member in the
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file structure if you want to point to a device structure
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release: called when the last reference to an open file is closed
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fsync: called by the fsync(2) system call
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fasync: called by the fcntl(2) system call when asynchronous
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(non-blocking) mode is enabled for a file
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Note that the file operations are implemented by the specific
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filesystem in which the inode resides. When opening a device node
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(character or block special) most filesystems will call special
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support routines in the VFS which will locate the required device
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driver information. These support routines replace the filesystem file
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operations with those for the device driver, and then proceed to call
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the new open() method for the file. This is how opening a device file
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in the filesystem eventually ends up calling the device driver open()
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method. Note the devfs (the Device FileSystem) has a more direct path
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from device node to device driver (this is an unofficial kernel
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patch).
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Directory Entry Cache (dcache) <section>
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------------------------------
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struct dentry_operations
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========================
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This describes how a filesystem can overload the standard dentry
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operations. Dentries and the dcache are the domain of the VFS and the
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individual filesystem implementations. Device drivers have no business
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here. These methods may be set to NULL, as they are either optional or
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the VFS uses a default. As of kernel 2.1.99, the following members are
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defined:
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struct dentry_operations {
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int (*d_revalidate)(struct dentry *);
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int (*d_hash) (struct dentry *, struct qstr *);
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int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
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void (*d_delete)(struct dentry *);
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void (*d_release)(struct dentry *);
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void (*d_iput)(struct dentry *, struct inode *);
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};
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d_revalidate: called when the VFS needs to revalidate a dentry. This
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is called whenever a name look-up finds a dentry in the
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dcache. Most filesystems leave this as NULL, because all their
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dentries in the dcache are valid
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d_hash: called when the VFS adds a dentry to the hash table
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d_compare: called when a dentry should be compared with another
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d_delete: called when the last reference to a dentry is
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deleted. This means no-one is using the dentry, however it is
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still valid and in the dcache
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d_release: called when a dentry is really deallocated
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d_iput: called when a dentry loses its inode (just prior to its
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being deallocated). The default when this is NULL is that the
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VFS calls iput(). If you define this method, you must call
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iput() yourself
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Each dentry has a pointer to its parent dentry, as well as a hash list
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of child dentries. Child dentries are basically like files in a
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directory.
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Directory Entry Cache APIs
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--------------------------
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There are a number of functions defined which permit a filesystem to
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manipulate dentries:
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dget: open a new handle for an existing dentry (this just increments
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the usage count)
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dput: close a handle for a dentry (decrements the usage count). If
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the usage count drops to 0, the "d_delete" method is called
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and the dentry is placed on the unused list if the dentry is
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still in its parents hash list. Putting the dentry on the
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unused list just means that if the system needs some RAM, it
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goes through the unused list of dentries and deallocates them.
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If the dentry has already been unhashed and the usage count
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drops to 0, in this case the dentry is deallocated after the
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"d_delete" method is called
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d_drop: this unhashes a dentry from its parents hash list. A
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subsequent call to dput() will dellocate the dentry if its
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usage count drops to 0
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d_delete: delete a dentry. If there are no other open references to
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the dentry then the dentry is turned into a negative dentry
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(the d_iput() method is called). If there are other
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references, then d_drop() is called instead
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d_add: add a dentry to its parents hash list and then calls
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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.
|
|
|
|
|
|
RCU-based dcache locking model
|
|
------------------------------
|
|
|
|
On many workloads, the most common operation on dcache is
|
|
to look up a dentry, given a parent dentry and the name
|
|
of the child. Typically, for every open(), stat() etc.,
|
|
the dentry corresponding to the pathname will be looked
|
|
up by walking the tree starting with the first component
|
|
of the pathname and using that dentry along with the next
|
|
component to look up the next level and so on. Since it
|
|
is a frequent operation for workloads like multiuser
|
|
environments and webservers, it is important to optimize
|
|
this path.
|
|
|
|
Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus
|
|
in every component during path look-up. Since 2.5.10 onwards,
|
|
fastwalk algorithm changed this by holding the dcache_lock
|
|
at the beginning and walking as many cached path component
|
|
dentries as possible. This signficantly decreases the number
|
|
of acquisition of dcache_lock. However it also increases the
|
|
lock hold time signficantly and affects performance in large
|
|
SMP machines. Since 2.5.62 kernel, dcache has been using
|
|
a new locking model that uses RCU to make dcache look-up
|
|
lock-free.
|
|
|
|
The current dcache locking model is not very different from the existing
|
|
dcache locking model. Prior to 2.5.62 kernel, dcache_lock
|
|
protected the hash chain, d_child, d_alias, d_lru lists as well
|
|
as d_inode and several other things like mount look-up. RCU-based
|
|
changes affect only the way the hash chain is protected. For everything
|
|
else the dcache_lock must be taken for both traversing as well as
|
|
updating. The hash chain updations too take the dcache_lock.
|
|
The significant change is the way d_lookup traverses the hash chain,
|
|
it doesn't acquire the dcache_lock for this and rely on RCU to
|
|
ensure that the dentry has not been *freed*.
|
|
|
|
|
|
Dcache locking details
|
|
----------------------
|
|
For many multi-user workloads, open() and stat() on files are
|
|
very frequently occurring operations. Both involve walking
|
|
of path names to find the dentry corresponding to the
|
|
concerned file. In 2.4 kernel, dcache_lock was held
|
|
during look-up of each path component. Contention and
|
|
cacheline bouncing of this global lock caused significant
|
|
scalability problems. With the introduction of RCU
|
|
in linux kernel, this was worked around by making
|
|
the look-up of path components during path walking lock-free.
|
|
|
|
|
|
Safe lock-free look-up of dcache hash table
|
|
===========================================
|
|
|
|
Dcache is a complex data structure with the hash table entries
|
|
also linked together in other lists. In 2.4 kernel, dcache_lock
|
|
protected all the lists. We applied RCU only on hash chain
|
|
walking. The rest of the lists are still protected by dcache_lock.
|
|
Some of the important changes are :
|
|
|
|
1. The deletion from hash chain is done using hlist_del_rcu() macro which
|
|
doesn't initialize next pointer of the deleted dentry and this
|
|
allows us to walk safely lock-free while a deletion is happening.
|
|
|
|
2. Insertion of a dentry into the hash table is done using
|
|
hlist_add_head_rcu() which take care of ordering the writes -
|
|
the writes to the dentry must be visible before the dentry
|
|
is inserted. This works in conjuction with hlist_for_each_rcu()
|
|
while walking the hash chain. The only requirement is that
|
|
all initialization to the dentry must be done before hlist_add_head_rcu()
|
|
since we don't have dcache_lock protection while traversing
|
|
the hash chain. This isn't different from the existing code.
|
|
|
|
3. The dentry looked up without holding dcache_lock by cannot be
|
|
returned for walking if it is unhashed. It then may have a NULL
|
|
d_inode or other bogosity since RCU doesn't protect the other
|
|
fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
|
|
indicate unhashed dentries and use this in conjunction with a
|
|
per-dentry lock (d_lock). Once looked up without the dcache_lock,
|
|
we acquire the per-dentry lock (d_lock) and check if the
|
|
dentry is unhashed. If so, the look-up is failed. If not, the
|
|
reference count of the dentry is increased and the dentry is returned.
|
|
|
|
4. Once a dentry is looked up, it must be ensured during the path
|
|
walk for that component it doesn't go away. In pre-2.5.10 code,
|
|
this was done holding a reference to the dentry. dcache_rcu does
|
|
the same. In some sense, dcache_rcu path walking looks like
|
|
the pre-2.5.10 version.
|
|
|
|
5. All dentry hash chain updations must take the dcache_lock as well as
|
|
the per-dentry lock in that order. dput() does this to ensure
|
|
that a dentry that has just been looked up in another CPU
|
|
doesn't get deleted before dget() can be done on it.
|
|
|
|
6. There are several ways to do reference counting of RCU protected
|
|
objects. One such example is in ipv4 route cache where
|
|
deferred freeing (using call_rcu()) is done as soon as
|
|
the reference count goes to zero. This cannot be done in
|
|
the case of dentries because tearing down of dentries
|
|
require blocking (dentry_iput()) which isn't supported from
|
|
RCU callbacks. Instead, tearing down of dentries happen
|
|
synchronously in dput(), but actual freeing happens later
|
|
when RCU grace period is over. This allows safe lock-free
|
|
walking of the hash chains, but a matched dentry may have
|
|
been partially torn down. The checking of DCACHE_UNHASHED
|
|
flag with d_lock held detects such dentries and prevents
|
|
them from being returned from look-up.
|
|
|
|
|
|
Maintaining POSIX rename semantics
|
|
==================================
|
|
|
|
Since look-up of dentries is lock-free, it can race against
|
|
a concurrent rename operation. For example, during rename
|
|
of file A to B, look-up of either A or B must succeed.
|
|
So, if look-up of B happens after A has been removed from the
|
|
hash chain but not added to the new hash chain, it may fail.
|
|
Also, a comparison while the name is being written concurrently
|
|
by a rename may result in false positive matches violating
|
|
rename semantics. Issues related to race with rename are
|
|
handled as described below :
|
|
|
|
1. Look-up can be done in two ways - d_lookup() which is safe
|
|
from simultaneous renames and __d_lookup() which is not.
|
|
If __d_lookup() fails, it must be followed up by a d_lookup()
|
|
to correctly determine whether a dentry is in the hash table
|
|
or not. d_lookup() protects look-ups using a sequence
|
|
lock (rename_lock).
|
|
|
|
2. The name associated with a dentry (d_name) may be changed if
|
|
a rename is allowed to happen simultaneously. To avoid memcmp()
|
|
in __d_lookup() go out of bounds due to a rename and false
|
|
positive comparison, the name comparison is done while holding the
|
|
per-dentry lock. This prevents concurrent renames during this
|
|
operation.
|
|
|
|
3. Hash table walking during look-up may move to a different bucket as
|
|
the current dentry is moved to a different bucket due to rename.
|
|
But we use hlists in dcache hash table and they are null-terminated.
|
|
So, even if a dentry moves to a different bucket, hash chain
|
|
walk will terminate. [with a list_head list, it may not since
|
|
termination is when the list_head in the original bucket is reached].
|
|
Since we redo the d_parent check and compare name while holding
|
|
d_lock, lock-free look-up will not race against d_move().
|
|
|
|
4. There can be a theoritical race when a dentry keeps coming back
|
|
to original bucket due to double moves. Due to this look-up may
|
|
consider that it has never moved and can end up in a infinite loop.
|
|
But this is not any worse that theoritical livelocks we already
|
|
have in the kernel.
|
|
|
|
|
|
Important guidelines for filesystem developers related to dcache_rcu
|
|
====================================================================
|
|
|
|
1. Existing dcache interfaces (pre-2.5.62) exported to filesystem
|
|
don't change. Only dcache internal implementation changes. However
|
|
filesystems *must not* delete from the dentry hash chains directly
|
|
using the list macros like allowed earlier. They must use dcache
|
|
APIs like d_drop() or __d_drop() depending on the situation.
|
|
|
|
2. d_flags is now protected by a per-dentry lock (d_lock). All
|
|
access to d_flags must be protected by it.
|
|
|
|
3. For a hashed dentry, checking of d_count needs to be protected
|
|
by d_lock.
|
|
|
|
|
|
Papers and other documentation on dcache locking
|
|
================================================
|
|
|
|
1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
|
|
|
|
2. http://lse.sourceforge.net/locking/dcache/dcache.html
|
|
|