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Subject[PATCH] VFS: update documentation
This patch brings the now out-of-date Documentation/filesystems/vfs.txt back
to life. Thanks to Carsten Otte, Trond Myklebust, and Anton Altaparmakov for
their help on updating this documentation.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
---

vfs.txt | 435 +++++++++++++++++++++++++++++++++++++++++++++++-----------------
1 files changed, 323 insertions(+), 112 deletions(-)

Index: 2.6/Documentation/filesystems/vfs.txt
===================================================================
--- 2.6.orig/Documentation/filesystems/vfs.txt
+++ 2.6/Documentation/filesystems/vfs.txt
@@ -1,35 +1,27 @@
-/* -*- auto-fill -*- */

- Overview of the Virtual File System
+ Overview of the Linux Virtual File System

- Richard Gooch <rgooch@atnf.csiro.au>
+ Original author: Richard Gooch <rgooch@atnf.csiro.au>

- 5-JUL-1999
+ Last updated on August 25, 2005

+ Copyright (C) 1999 Richard Gooch
+ Copyright (C) 2005 Pekka Enberg

-Conventions used in this document <section>
-=================================
+ This file is released under the GPLv2.

-Each section in this document will have the string "<section>" at the
-right-hand side of the section title. Each subsection will have
-"<subsection>" at the right-hand side. These strings are meant to make
-it easier to search through the document.

-NOTE that the master copy of this document is available online at:
-http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt
-
-
-What is it? <section>
+What is it?
===========

The Virtual File System (otherwise known as the Virtual Filesystem
Switch) is the software layer in the kernel that provides the
filesystem interface to userspace programs. It also provides an
abstraction within the kernel which allows different filesystem
-implementations to co-exist.
+implementations to coexist.


-A Quick Look At How It Works <section>
+A Quick Look At How It Works
============================

In this section I'll briefly describe how things work, before
@@ -38,7 +30,8 @@ when user programs open and manipulate f
other view which is how a filesystem is supported and subsequently
mounted.

-Opening a File <subsection>
+
+Opening a File
--------------

The VFS implements the open(2), stat(2), chmod(2) and similar system
@@ -77,7 +70,7 @@ back to userspace.

Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file
-descriptors). The freshly allocated file structure is initialised with
+descriptors). The freshly allocated file structure is initialized with
a pointer to the dentry and a set of file operation member functions.
These are taken from the inode data. The open() file method is then
called so the specific filesystem implementation can do it's work. You
@@ -102,7 +95,8 @@ filesystem or driver code at the same ti
processors. You should ensure that access to shared resources is
protected by appropriate locks.

-Registering and Mounting a Filesystem <subsection>
+
+Registering and Mounting a Filesystem
-------------------------------------

If you want to support a new kind of filesystem in the kernel, all you
@@ -123,17 +117,21 @@ updated to point to the root inode for t
It's now time to look at things in more detail.


-struct file_system_type <section>
+struct file_system_type
=======================

-This describes the filesystem. As of kernel 2.1.99, the following
+This describes the filesystem. As of kernel 2.6.13, the following
members are defined:

struct file_system_type {
const char *name;
int fs_flags;
- struct super_block *(*read_super) (struct super_block *, void *, int);
- struct file_system_type * next;
+ struct super_block *(*get_sb) (struct file_system_type *, int,
+ const char *, void *);
+ void (*kill_sb) (struct super_block *);
+ struct module *owner;
+ struct file_system_type * next;
+ struct list_head fs_supers;
};

name: the name of the filesystem type, such as "ext2", "iso9660",
@@ -141,51 +139,97 @@ struct file_system_type {

fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)

- read_super: the method to call when a new instance of this
+ get_sb: the method to call when a new instance of this
filesystem should be mounted

- next: for internal VFS use: you should initialise this to NULL
+ kill_sb: the method to call when an instance of this filesystem
+ should be unmounted
+
+ owner: for internal VFS use: you should initialize this to THIS_MODULE in
+ most cases.

-The read_super() method has the following arguments:
+ next: for internal VFS use: you should initialize this to NULL
+
+The get_sb() method has the following arguments:

struct super_block *sb: the superblock structure. This is partially
- initialised by the VFS and the rest must be initialised by the
- read_super() method
+ initialized by the VFS and the rest must be initialized by the
+ get_sb() method
+
+ int flags: mount flags
+
+ const char *dev_name: the device name we are mounting.

void *data: arbitrary mount options, usually comes as an ASCII
string

int silent: whether or not to be silent on error

-The read_super() method must determine if the block device specified
+The get_sb() method must determine if the block device specified
in the superblock contains a filesystem of the type the method
supports. On success the method returns the superblock pointer, on
failure it returns NULL.

The most interesting member of the superblock structure that the
-read_super() method fills in is the "s_op" field. This is a pointer to
+get_sb() method fills in is the "s_op" field. This is a pointer to
a "struct super_operations" which describes the next level of the
filesystem implementation.

+Usually, a filesystem uses generic one of the generic get_sb()
+implementations and provides a fill_super() method instead. The
+generic methods are:
+
+ get_sb_bdev: mount a filesystem residing on a block device

-struct super_operations <section>
+ get_sb_nodev: mount a filesystem that is not backed by a device
+
+ get_sb_single: mount a filesystem which shares the instance between
+ all mounts
+
+A fill_super() method implementation has the following arguments:
+
+ struct super_block *sb: the superblock structure. The method fill_super()
+ must initialize this properly.
+
+ void *data: arbitrary mount options, usually comes as an ASCII
+ string
+
+ int silent: whether or not to be silent on error
+
+
+struct super_operations
=======================

This describes how the VFS can manipulate the superblock of your
-filesystem. As of kernel 2.1.99, the following members are defined:
+filesystem. As of kernel 2.6.13, the following members are defined:

struct super_operations {
- void (*read_inode) (struct inode *);
- int (*write_inode) (struct inode *, int);
- void (*put_inode) (struct inode *);
- void (*drop_inode) (struct inode *);
- void (*delete_inode) (struct inode *);
- int (*notify_change) (struct dentry *, struct iattr *);
- void (*put_super) (struct super_block *);
- void (*write_super) (struct super_block *);
- int (*statfs) (struct super_block *, struct statfs *, int);
- int (*remount_fs) (struct super_block *, int *, char *);
- void (*clear_inode) (struct inode *);
+ struct inode *(*alloc_inode)(struct super_block *sb);
+ void (*destroy_inode)(struct inode *);
+
+ void (*read_inode) (struct inode *);
+
+ void (*dirty_inode) (struct inode *);
+ int (*write_inode) (struct inode *, int);
+ void (*put_inode) (struct inode *);
+ void (*drop_inode) (struct inode *);
+ void (*delete_inode) (struct inode *);
+ void (*put_super) (struct super_block *);
+ void (*write_super) (struct super_block *);
+ int (*sync_fs)(struct super_block *sb, int wait);
+ void (*write_super_lockfs) (struct super_block *);
+ void (*unlockfs) (struct super_block *);
+ int (*statfs) (struct super_block *, struct kstatfs *);
+ int (*remount_fs) (struct super_block *, int *, char *);
+ void (*clear_inode) (struct inode *);
+ void (*umount_begin) (struct super_block *);
+
+ void (*sync_inodes) (struct super_block *sb,
+ struct writeback_control *wbc);
+ int (*show_options)(struct seq_file *, struct vfsmount *);
+
+ ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
+ ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
};

All methods are called without any locks being held, unless otherwise
@@ -193,43 +237,62 @@ noted. This means that most methods can
only called from a process context (i.e. not from an interrupt handler
or bottom half).

+ alloc_inode: this method is called by inode_alloc() to allocate memory
+ for struct inode and initialize it.
+
+ destroy_inode: this method is called by destroy_inode() to release
+ resources allocated for struct inode.
+
read_inode: this method is called to read a specific inode from the
- mounted filesystem. The "i_ino" member in the "struct inode"
- will be initialised by the VFS to indicate which inode to
- read. Other members are filled in by this method
+ mounted filesystem. The i_ino member in the struct inode is
+ initialized by the VFS to indicate which inode to read. Other
+ members are filled in by this method.
+
+ You can set this to NULL and use iget5_locked() instead of iget()
+ to read inodes. This is necessary for filesystems for which the
+ inode number is not sufficient to identify an inode.
+
+ dirty_inode: this method is called by the VFS to mark an inode dirty.

write_inode: this method is called when the VFS needs to write an
inode to disc. The second parameter indicates whether the write
should be synchronous or not, not all filesystems check this flag.

put_inode: called when the VFS inode is removed from the inode
- cache. This method is optional
+ cache.

drop_inode: called when the last access to the inode is dropped,
with the inode_lock spinlock held.

- This method should be either NULL (normal unix filesystem
+ This method should be either NULL (normal UNIX filesystem
semantics) or "generic_delete_inode" (for filesystems that do not
want to cache inodes - causing "delete_inode" to always be
called regardless of the value of i_nlink)

- The "generic_delete_inode()" behaviour is equivalent to the
+ The "generic_delete_inode()" behavior is equivalent to the
old practice of using "force_delete" in the put_inode() case,
but does not have the races that the "force_delete()" approach
had.

delete_inode: called when the VFS wants to delete an inode

- notify_change: called when VFS inode attributes are changed. If this
- is NULL the VFS falls back to the write_inode() method. This
- is called with the kernel lock held
-
put_super: called when the VFS wishes to free the superblock
(i.e. unmount). This is called with the superblock lock held

write_super: called when the VFS superblock needs to be written to
disc. This method is optional

+ sync_fs: called when VFS is writing out all dirty data associated with
+ a superblock. The second parameter indicates whether the method
+ should wait until the write out has been completed. Optional.
+
+ write_super_lockfs: called when VFS is locking a filesystem and forcing
+ it into a consistent state. This function is currently used by the
+ Logical Volume Manager (LVM).
+
+ unlockfs: called when VFS is unlocking a filesystem and making it writable
+ again.
+
statfs: called when the VFS needs to get filesystem statistics. This
is called with the kernel lock held

@@ -238,21 +301,31 @@ or bottom half).

clear_inode: called then the VFS clears the inode. Optional

+ umount_begin: called when the VFS is unmounting a filesystem.
+
+ sync_inodes: called when the VFS is writing out dirty data associated with
+ a superblock.
+
+ show_options: called by the VFS to show mount options for /proc/<pid>/mounts.
+
+ quota_read: called by the VFS to read from filesystem quota file.
+
+ quota_write: called by the VFS to write to filesystem quota file.
+
The read_inode() method is responsible for filling in the "i_op"
field. This is a pointer to a "struct inode_operations" which
describes the methods that can be performed on individual inodes.


-struct inode_operations <section>
+struct inode_operations
=======================

This describes how the VFS can manipulate an inode in your
-filesystem. As of kernel 2.1.99, the following members are defined:
+filesystem. As of kernel 2.6.13, the following members are defined:

struct inode_operations {
- struct file_operations * default_file_ops;
- int (*create) (struct inode *,struct dentry *,int);
- int (*lookup) (struct inode *,struct dentry *);
+ int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
+ struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
int (*link) (struct dentry *,struct inode *,struct dentry *);
int (*unlink) (struct inode *,struct dentry *);
int (*symlink) (struct inode *,struct dentry *,const char *);
@@ -261,25 +334,22 @@ struct inode_operations {
int (*mknod) (struct inode *,struct dentry *,int,dev_t);
int (*rename) (struct inode *, struct dentry *,
struct inode *, struct dentry *);
- int (*readlink) (struct dentry *, char *,int);
- struct dentry * (*follow_link) (struct dentry *, struct dentry *);
- int (*readpage) (struct file *, struct page *);
- int (*writepage) (struct page *page, struct writeback_control *wbc);
- int (*bmap) (struct inode *,int);
+ int (*readlink) (struct dentry *, char __user *,int);
+ void * (*follow_link) (struct dentry *, struct nameidata *);
+ void (*put_link) (struct dentry *, struct nameidata *, void *);
void (*truncate) (struct inode *);
- int (*permission) (struct inode *, int);
- int (*smap) (struct inode *,int);
- int (*updatepage) (struct file *, struct page *, const char *,
- unsigned long, unsigned int, int);
- int (*revalidate) (struct dentry *);
+ int (*permission) (struct inode *, int, struct nameidata *);
+ int (*setattr) (struct dentry *, struct iattr *);
+ int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
+ int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
+ ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
+ ssize_t (*listxattr) (struct dentry *, char *, size_t);
+ int (*removexattr) (struct dentry *, const char *);
};

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

- default_file_ops: this is a pointer to a "struct file_operations"
- which describes how to open and then manipulate open files
-
create: called by the open(2) and creat(2) system calls. Only
required if you want to support regular files. The dentry you
get should not have an inode (i.e. it should be a negative
@@ -328,31 +398,143 @@ otherwise noted.
you want to support reading symbolic links

follow_link: called by the VFS to follow a symbolic link to the
- inode it points to. Only required if you want to support
- symbolic links
+ inode it points to. Only required if you want to support
+ symbolic links. This function returns a void pointer cookie
+ that is passed to put_link().
+
+ put_link: called by the VFS to release resources allocated by
+ follow_link(). The cookie returned by follow_link() is passed to
+ to this function as the last parameter. It is used by filesystems
+ such as NFS where page cache is not stable (i.e. page that was
+ installed when the symbolic link walk started might not be in the
+ page cache at the end of the walk).
+
+ truncate: called by the VFS to change the size of a file. The i_size
+ field of the inode is set to the desired size by the VFS before
+ this function is called. This function is called by the truncate(2)
+ system call and related functionality.
+
+ permission: called by the VFS to check for access rights on a POSIX-like
+ filesystem.
+
+ setattr: called by the VFS to set attributes for a file. This function is
+ called by chmod(2) and related system calls.
+
+ getattr: called by the VFS to get attributes of a file. This function is
+ called by stat(2) and related system calls.
+
+ setxattr: called by the VFS to set an extended attribute for a file.
+ Extended attribute is a name:value pair associated with an inode. This
+ function is called by setxattr(2) system call.
+
+ getxattr: called by the VFS to retrieve the value of an extended attribute
+ name. This function is called by getxattr(2) function call.
+
+ listxattr: called by the VFS to list all extended attributes for a given
+ file. This function is called by listxattr(2) system call.
+
+ removexattr: called by the VFS to remove an extended attribute from a file.
+ This function is called by removexattr(2) system call.
+
+
+struct address_space_operations
+===============================
+
+This describes how the VFS can manipulate mapping of a file to page cache in
+your filesystem. As of kernel 2.6.13, 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);
+};
+
+ writepage: called by the VM write a dirty page to backing store.
+
+ readpage: called by the VM to read a page from backing store.
+
+ 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.
+
+ writepages: called by the VM to write out pages associated with the
+ address_space object.
+
+ set_page_dirty: called by the VM to set a page dirty.
+
+ readpages: called by the VM to read pages associated with the address_space
+ object.

+ prepare_write: called by the generic write path in VM to set up a write
+ request for a page.

-struct file_operations <section>
+ commit_write: called by the generic write path in VM to write page to
+ its backing store.
+
+ bmap: called by the VFS to map a logical block offset within object to
+ physical block number. This method is use by for the legacy FIBMAP
+ ioctl. Other uses are discouraged.
+
+ invalidatepage: called by the VM on truncate to disassociate a page from its
+ address_space mapping.
+
+ releasepage: called by the VFS to release filesystem specific metadata from
+ a page.
+
+ direct_IO: called by the VM for direct I/O writes and reads.
+
+ 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.
+
+
+struct file_operations
======================

This describes how the VFS can manipulate an open file. As of kernel
-2.1.99, the following members are defined:
+2.6.13, the following members are defined:

struct file_operations {
loff_t (*llseek) (struct file *, loff_t, int);
- ssize_t (*read) (struct file *, char *, size_t, loff_t *);
- ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
+ ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
+ ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
+ ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
+ ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);
int (*readdir) (struct file *, void *, filldir_t);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
+ 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 (*fasync) (struct file *, int);
- int (*check_media_change) (kdev_t dev);
- int (*revalidate) (kdev_t dev);
+ 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 *);
};

Again, all methods are called without any locks being held, unless
@@ -362,8 +544,12 @@ otherwise noted.

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
@@ -372,18 +558,25 @@ otherwise noted.

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

@@ -392,6 +585,23 @@ otherwise noted.
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 readv(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
+
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
@@ -400,29 +610,28 @@ driver information. These support routin
operations with those for the device driver, and then proceed to call
the new open() method for the file. This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
-method. Note the devfs (the Device FileSystem) has a more direct path
-from device node to device driver (this is an unofficial kernel
-patch).
+method.


-Directory Entry Cache (dcache) <section>
-------------------------------
+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 2.1.99, the following members are
+the VFS uses a default. As of kernel 2.6.13, the following members are
defined:

struct dentry_operations {
- int (*d_revalidate)(struct dentry *);
+ int (*d_revalidate)(struct dentry *, struct nameidata *);
int (*d_hash) (struct dentry *, struct qstr *);
int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
- void (*d_delete)(struct dentry *);
+ int (*d_delete)(struct dentry *);
void (*d_release)(struct dentry *);
void (*d_iput)(struct dentry *, struct inode *);
};
@@ -451,6 +660,7 @@ Each dentry has a pointer to its parent
of child dentries. Child dentries are basically like files in a
directory.

+
Directory Entry Cache APIs
--------------------------

@@ -471,7 +681,7 @@ manipulate dentries:
"d_delete" method is called

d_drop: this unhashes a dentry from its parents hash list. A
- subsequent call to dput() will dellocate the dentry if its
+ 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
@@ -507,16 +717,16 @@ up by walking the tree starting with the
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
+environments and web servers, 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
+fast-walk 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
+dentries as possible. This significantly decreases the number
of acquisition of dcache_lock. However it also increases the
-lock hold time signficantly and affects performance in large
+lock hold time significantly 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.
@@ -527,7 +737,7 @@ protected the hash chain, d_child, d_ali
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.
+updating. The hash chain updates 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*.
@@ -535,14 +745,15 @@ ensure that the dentry has not been *fre

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
+cache-line bouncing of this global lock caused significant
scalability problems. With the introduction of RCU
-in linux kernel, this was worked around by making
+in Linux kernel, this was worked around by making
the look-up of path components during path walking lock-free.


@@ -562,7 +773,7 @@ Some of the important changes are :
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()
+ is inserted. This works in conjunction 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
@@ -584,7 +795,7 @@ Some of the important changes are :
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
+5. All dentry hash chain updates 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.
@@ -640,10 +851,10 @@ handled as described below :
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
+4. There can be a theoretical 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
+ But this is not any worse that theoretical livelocks we already
have in the kernel.


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