Commit 0eeca283 authored by Robert Love's avatar Robert Love Committed by Linus Torvalds
Browse files

[PATCH] inotify

inotify is intended to correct the deficiencies of dnotify, particularly
its inability to scale and its terrible user interface:

        * dnotify requires the opening of one fd per each directory
          that you intend to watch. This quickly results in too many
          open files and pins removable media, preventing unmount.
        * dnotify is directory-based. You only learn about changes to
          directories. Sure, a change to a file in a directory affects
          the directory, but you are then forced to keep a cache of
          stat structures.
        * dnotify's interface to user-space is awful.  Signals?

inotify provides a more usable, simple, powerful solution to file change

        * inotify's interface is a system call that returns a fd, not SIGIO.
	  You get a single fd, which is select()-able.
        * inotify has an event that says "the filesystem that the item
          you were watching is on was unmounted."
        * inotify can watch directories or files.

Inotify is currently used by Beagle (a desktop search infrastructure),
Gamin (a FAM replacement), and other projects.

See Documentation/filesystems/inotify.txt.
Signed-off-by: default avatarRobert Love <>
Cc: John McCutchan <>
Cc: Christoph Hellwig <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent bd4c625c
a powerful yet simple file change notification system
Document started 15 Mar 2005 by Robert Love <>
(i) User Interface
Inotify is controlled by a set of three sys calls
First step in using inotify is to initialise an inotify instance
int fd = inotify_init ();
Change events are managed by "watches". A watch is an (object,mask) pair where
the object is a file or directory and the mask is a bit mask of one or more
inotify events that the application wishes to receive. See <linux/inotify.h>
for valid events. A watch is referenced by a watch descriptor, or wd.
Watches are added via a path to the file.
Watches on a directory will return events on any files inside of the directory.
Adding a watch is simple,
int wd = inotify_add_watch (fd, path, mask);
You can add a large number of files via something like
for each file to watch {
int wd = inotify_add_watch (fd, file, mask);
You can update an existing watch in the same manner, by passing in a new mask.
An existing watch is removed via the INOTIFY_IGNORE ioctl, for example
inotify_rm_watch (fd, wd);
Events are provided in the form of an inotify_event structure that is read(2)
from a inotify instance fd. The filename is of dynamic length and follows the
struct. It is of size len. The filename is padded with null bytes to ensure
proper alignment. This padding is reflected in len.
You can slurp multiple events by passing a large buffer, for example
size_t len = read (fd, buf, BUF_LEN);
Will return as many events as are available and fit in BUF_LEN.
each inotify instance fd is also select()- and poll()-able.
You can find the size of the current event queue via the FIONREAD ioctl.
All watches are destroyed and cleaned up on close.
(ii) Internal Kernel Implementation
Each open inotify instance is associated with an inotify_device structure.
Each watch is associated with an inotify_watch structure. Watches are chained
off of each associated device and each associated inode.
See fs/inotify.c for the locking and lifetime rules.
(iii) Rationale
Q: What is the design decision behind not tying the watch to the open fd of
the watched object?
A: Watches are associated with an open inotify device, not an open file.
This solves the primary problem with dnotify: keeping the file open pins
the file and thus, worse, pins the mount. Dnotify is therefore infeasible
for use on a desktop system with removable media as the media cannot be
Q: What is the design decision behind using an-fd-per-device as opposed to
an fd-per-watch?
A: An fd-per-watch quickly consumes more file descriptors than are allowed,
more fd's than are feasible to manage, and more fd's than are optimally
select()-able. Yes, root can bump the per-process fd limit and yes, users
can use epoll, but requiring both is a silly and extraneous requirement.
A watch consumes less memory than an open file, separating the number
spaces is thus sensible. The current design is what user-space developers
want: Users initialize inotify, once, and add n watches, requiring but one fd
and no twiddling with fd limits. Initializing an inotify instance two
thousand times is silly. If we can implement user-space's preferences
cleanly--and we can, the idr layer makes stuff like this trivial--then we
There are other good arguments. With a single fd, there is a single
item to block on, which is mapped to a single queue of events. The single
fd returns all watch events and also any potential out-of-band data. If
every fd was a separate watch,
- There would be no way to get event ordering. Events on file foo and
file bar would pop poll() on both fd's, but there would be no way to tell
which happened first. A single queue trivially gives you ordering. Such
ordering is crucial to existing applications such as Beagle. Imagine
"mv a b ; mv b a" events without ordering.
- We'd have to maintain n fd's and n internal queues with state,
versus just one. It is a lot messier in the kernel. A single, linear
queue is the data structure that makes sense.
- User-space developers prefer the current API. The Beagle guys, for
example, love it. Trust me, I asked. It is not a surprise: Who'd want
to manage and block on 1000 fd's via select?
- You'd have to manage the fd's, as an example: Call close() when you
received a delete event.
- No way to get out of band data.
- 1024 is still too low. ;-)
When you talk about designing a file change notification system that
scales to 1000s of directories, juggling 1000s of fd's just does not seem
the right interface. It is too heavy.
Q: Why the system call approach?
A: The poor user-space interface is the second biggest problem with dnotify.
Signals are a terrible, terrible interface for file notification. Or for
anything, for that matter. The ideal solution, from all perspectives, is a
file descriptor-based one that allows basic file I/O and poll/select.
Obtaining the fd and managing the watches could have been done either via a
device file or a family of new system calls. We decided to implement a
family of system calls because that is the preffered approach for new kernel
features and it means our user interface requirements.
Additionally, it _is_ possible to more than one instance and
juggle more than one queue and thus more than one associated fd.
...@@ -291,3 +291,6 @@ ENTRY(sys_call_table) ...@@ -291,3 +291,6 @@ ENTRY(sys_call_table)
.long sys_keyctl .long sys_keyctl
.long sys_ioprio_set .long sys_ioprio_set
.long sys_ioprio_get /* 290 */ .long sys_ioprio_get /* 290 */
.long sys_inotify_init
.long sys_inotify_add_watch
.long sys_inotify_rm_watch
...@@ -359,6 +359,19 @@ config ROMFS_FS ...@@ -359,6 +359,19 @@ config ROMFS_FS
If you don't know whether you need it, then you don't need it: If you don't know whether you need it, then you don't need it:
answer N. answer N.
config INOTIFY
bool "Inotify file change notification support"
default y
Say Y here to enable inotify support and the /dev/inotify character
device. Inotify is a file change notification system and a
replacement for dnotify. Inotify fixes numerous shortcomings in
dnotify and introduces several new features. It allows monitoring
of both files and directories via a single open fd. Multiple file
events are supported.
If unsure, say Y.
config QUOTA config QUOTA
bool "Quota support" bool "Quota support"
help help
...@@ -12,6 +12,7 @@ obj-y := open.o read_write.o file_table.o buffer.o bio.o super.o \ ...@@ -12,6 +12,7 @@ obj-y := open.o read_write.o file_table.o buffer.o bio.o super.o \
seq_file.o xattr.o libfs.o fs-writeback.o mpage.o direct-io.o \ seq_file.o xattr.o libfs.o fs-writeback.o mpage.o direct-io.o \
ioprio.o ioprio.o
obj-$(CONFIG_INOTIFY) += inotify.o
obj-$(CONFIG_EPOLL) += eventpoll.o obj-$(CONFIG_EPOLL) += eventpoll.o
obj-$(CONFIG_COMPAT) += compat.o obj-$(CONFIG_COMPAT) += compat.o
...@@ -10,7 +10,7 @@ ...@@ -10,7 +10,7 @@
#include <linux/mm.h> #include <linux/mm.h>
#include <linux/string.h> #include <linux/string.h>
#include <linux/smp_lock.h> #include <linux/smp_lock.h>
#include <linux/dnotify.h> #include <linux/fsnotify.h>
#include <linux/fcntl.h> #include <linux/fcntl.h>
#include <linux/quotaops.h> #include <linux/quotaops.h>
#include <linux/security.h> #include <linux/security.h>
...@@ -107,31 +107,8 @@ int inode_setattr(struct inode * inode, struct iattr * attr) ...@@ -107,31 +107,8 @@ int inode_setattr(struct inode * inode, struct iattr * attr)
out: out:
return error; return error;
} }
EXPORT_SYMBOL(inode_setattr); EXPORT_SYMBOL(inode_setattr);
int setattr_mask(unsigned int ia_valid)
unsigned long dn_mask = 0;
if (ia_valid & ATTR_UID)
dn_mask |= DN_ATTRIB;
if (ia_valid & ATTR_GID)
dn_mask |= DN_ATTRIB;
if (ia_valid & ATTR_SIZE)
dn_mask |= DN_MODIFY;
/* both times implies a utime(s) call */
dn_mask |= DN_ATTRIB;
else if (ia_valid & ATTR_ATIME)
dn_mask |= DN_ACCESS;
else if (ia_valid & ATTR_MTIME)
dn_mask |= DN_MODIFY;
if (ia_valid & ATTR_MODE)
dn_mask |= DN_ATTRIB;
return dn_mask;
int notify_change(struct dentry * dentry, struct iattr * attr) int notify_change(struct dentry * dentry, struct iattr * attr)
{ {
struct inode *inode = dentry->d_inode; struct inode *inode = dentry->d_inode;
...@@ -197,11 +174,9 @@ int notify_change(struct dentry * dentry, struct iattr * attr) ...@@ -197,11 +174,9 @@ int notify_change(struct dentry * dentry, struct iattr * attr)
if (ia_valid & ATTR_SIZE) if (ia_valid & ATTR_SIZE)
up_write(&dentry->d_inode->i_alloc_sem); up_write(&dentry->d_inode->i_alloc_sem);
if (!error) { if (!error)
unsigned long dn_mask = setattr_mask(ia_valid); fsnotify_change(dentry, ia_valid);
if (dn_mask)
dnotify_parent(dentry, dn_mask);
return error; return error;
} }
...@@ -37,7 +37,7 @@ ...@@ -37,7 +37,7 @@
#include <linux/ctype.h> #include <linux/ctype.h>
#include <linux/module.h> #include <linux/module.h>
#include <linux/dirent.h> #include <linux/dirent.h>
#include <linux/dnotify.h> #include <linux/fsnotify.h>
#include <linux/highuid.h> #include <linux/highuid.h>
#include <linux/sunrpc/svc.h> #include <linux/sunrpc/svc.h>
#include <linux/nfsd/nfsd.h> #include <linux/nfsd/nfsd.h>
...@@ -1307,9 +1307,13 @@ static ssize_t compat_do_readv_writev(int type, struct file *file, ...@@ -1307,9 +1307,13 @@ static ssize_t compat_do_readv_writev(int type, struct file *file,
out: out:
if (iov != iovstack) if (iov != iovstack)
kfree(iov); kfree(iov);
if ((ret + (type == READ)) > 0) if ((ret + (type == READ)) > 0) {
dnotify_parent(file->f_dentry, struct dentry *dentry = file->f_dentry;
(type == READ) ? DN_ACCESS : DN_MODIFY); if (type == READ)
return ret; return ret;
} }
...@@ -16,6 +16,7 @@ ...@@ -16,6 +16,7 @@
#include <linux/eventpoll.h> #include <linux/eventpoll.h>
#include <linux/mount.h> #include <linux/mount.h>
#include <linux/cdev.h> #include <linux/cdev.h>
#include <linux/fsnotify.h>
/* sysctl tunables... */ /* sysctl tunables... */
struct files_stat_struct files_stat = { struct files_stat_struct files_stat = {
...@@ -126,6 +127,8 @@ void fastcall __fput(struct file *file) ...@@ -126,6 +127,8 @@ void fastcall __fput(struct file *file)
struct inode *inode = dentry->d_inode; struct inode *inode = dentry->d_inode;
might_sleep(); might_sleep();
/* /*
* The function eventpoll_release() should be the first called * The function eventpoll_release() should be the first called
* in the file cleanup chain. * in the file cleanup chain.
...@@ -21,6 +21,7 @@ ...@@ -21,6 +21,7 @@
#include <linux/pagemap.h> #include <linux/pagemap.h>
#include <linux/cdev.h> #include <linux/cdev.h>
#include <linux/bootmem.h> #include <linux/bootmem.h>
#include <linux/inotify.h>
/* /*
* This is needed for the following functions: * This is needed for the following functions:
...@@ -202,6 +203,10 @@ void inode_init_once(struct inode *inode) ...@@ -202,6 +203,10 @@ void inode_init_once(struct inode *inode)
INIT_LIST_HEAD(&inode->i_data.i_mmap_nonlinear); INIT_LIST_HEAD(&inode->i_data.i_mmap_nonlinear);
spin_lock_init(&inode->i_lock); spin_lock_init(&inode->i_lock);
i_size_ordered_init(inode); i_size_ordered_init(inode);
sema_init(&inode->inotify_sem, 1);
} }
EXPORT_SYMBOL(inode_init_once); EXPORT_SYMBOL(inode_init_once);
...@@ -351,6 +356,7 @@ int invalidate_inodes(struct super_block * sb) ...@@ -351,6 +356,7 @@ int invalidate_inodes(struct super_block * sb)
down(&iprune_sem); down(&iprune_sem);
spin_lock(&inode_lock); spin_lock(&inode_lock);
busy = invalidate_list(&sb->s_inodes, &throw_away); busy = invalidate_list(&sb->s_inodes, &throw_away);
spin_unlock(&inode_lock); spin_unlock(&inode_lock);
* fs/inotify.c - inode-based file event notifications
* Authors:
* John McCutchan <>
* Robert Love <>
* Copyright (C) 2005 John McCutchan
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License as published by the
* Free Software Foundation; either version 2, or (at your option) any
* later version.
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* General Public License for more details.
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/idr.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/poll.h>
#include <linux/device.h>
#include <linux/miscdevice.h>
#include <linux/init.h>
#include <linux/list.h>
#include <linux/writeback.h>
#include <linux/inotify.h>
#include <asm/ioctls.h>
static atomic_t inotify_cookie;
static kmem_cache_t *watch_cachep;
static kmem_cache_t *event_cachep;
static struct vfsmount *inotify_mnt;
/* These are configurable via /proc/sys/inotify */
int inotify_max_user_devices;
int inotify_max_user_watches;
int inotify_max_queued_events;
* Lock ordering:
* dentry->d_lock (used to keep d_move() away from dentry->d_parent)
* iprune_sem (synchronize shrink_icache_memory())
* inode_lock (protects the super_block->s_inodes list)
* inode->inotify_sem (protects inode->inotify_watches and watches->i_list)
* inotify_dev->sem (protects inotify_device and watches->d_list)
* Lifetimes of the three main data structures--inotify_device, inode, and
* inotify_watch--are managed by reference count.
* inotify_device: Lifetime is from open until release. Additional references
* can bump the count via get_inotify_dev() and drop the count via
* put_inotify_dev().
* inotify_watch: Lifetime is from create_watch() to destory_watch().
* Additional references can bump the count via get_inotify_watch() and drop
* the count via put_inotify_watch().
* inode: Pinned so long as the inode is associated with a watch, from
* create_watch() to put_inotify_watch().
* struct inotify_device - represents an open instance of an inotify device
* This structure is protected by the semaphore 'sem'.
struct inotify_device {
wait_queue_head_t wq; /* wait queue for i/o */
struct idr idr; /* idr mapping wd -> watch */
struct semaphore sem; /* protects this bad boy */
struct list_head events; /* list of queued events */
struct list_head watches; /* list of watches */
atomic_t count; /* reference count */
struct user_struct *user; /* user who opened this dev */
unsigned int queue_size; /* size of the queue (bytes) */
unsigned int event_count; /* number of pending events */
unsigned int max_events; /* maximum number of events */
* struct inotify_kernel_event - An inotify event, originating from a watch and
* queued for user-space. A list of these is attached to each instance of the
* device. In read(), this list is walked and all events that can fit in the
* buffer are returned.
* Protected by dev->sem of the device in which we are queued.
struct inotify_kernel_event {
struct inotify_event event; /* the user-space event */
struct list_head list; /* entry in inotify_device's list */
char *name; /* filename, if any */
* struct inotify_watch - represents a watch request on a specific inode
* d_list is protected by dev->sem of the associated watch->dev.
* i_list and mask are protected by inode->inotify_sem of the associated inode.
* dev, inode, and wd are never written to once the watch is created.
struct inotify_watch {
struct list_head d_list; /* entry in inotify_device's list */
struct list_head i_list; /* entry in inode's list */
atomic_t count; /* reference count */
struct inotify_device *dev; /* associated device */
struct inode *inode; /* associated inode */
s32 wd; /* watch descriptor */
u32 mask; /* event mask for this watch */
static inline void get_inotify_dev(struct inotify_device *dev)
static inline void put_inotify_dev(struct inotify_device *dev)
if (atomic_dec_and_test(&dev->count)) {
static inline void get_inotify_watch(struct inotify_watch *watch)
* put_inotify_watch - decrements the ref count on a given watch. cleans up
* the watch and its references if the count reaches zero.
static inline void put_inotify_watch(struct inotify_watch *watch)
if (atomic_dec_and_test(&watch->count)) {
kmem_cache_free(watch_cachep, watch);
* kernel_event - create a new kernel event with the given parameters
* This function can sleep.
static struct inotify_kernel_event * kernel_event(s32 wd, u32 mask, u32 cookie,
const char *name)
struct inotify_kernel_event *kevent;
kevent = kmem_cache_alloc(event_cachep, GFP_KERNEL);
if (unlikely(!kevent))
return NULL;
/* we hand this out to user-space, so zero it just in case */
memset(&kevent->event, 0, sizeof(struct inotify_event));
kevent->event.wd = wd;
kevent->event.mask = mask;
kevent->event.cookie = cookie;
if (name) {
size_t len, rem, event_size = sizeof(struct inotify_event);
* We need to pad the filename so as to properly align an
* array of inotify_event structures. Because the structure is
* small and the common case is a small filename, we just round
* up to the next multiple of the structure's sizeof. This is
* simple and safe for all architectures.
len = strlen(name) + 1;
rem = event_size - len;
if (len > event_size) {
rem = event_size - (len % event_size);
if (len % event_size == 0)
rem = 0;
kevent->name = kmalloc(len + rem, GFP_KERNEL);
if (unlikely(!kevent->name)) {
kmem_cache_free(event_cachep, kevent);
return NULL;
memcpy(kevent->name, name, len);
if (rem)
memset(kevent->name + len, 0, rem);
kevent->event.len = len + rem;
} else {
kevent->event.len = 0;
kevent->name = NULL;
return kevent;
* inotify_dev_get_event - return the next event in the given dev's queue
* Caller must hold dev->sem.
static inline struct inotify_kernel_event *
inotify_dev_get_event(struct inotify_device *dev)
return list_entry(dev->, struct inotify_kernel_event, list);
* inotify_dev_queue_event - add a new event to the given device
* Caller must hold dev->sem. Can sleep (calls kernel_event()).
static void inotify_dev_queue_event(struct inotify_device *dev,
struct inotify_watch *watch, u32 mask,
u32 cookie, const char *name)
struct inotify_kernel_event *kevent, *last;
/* coalescing: drop this event if it is a dupe of the previous */
last = inotify_dev_get_event(dev);
if (last && last->event.mask == mask && last->event.wd == watch->wd &&
last->event.cookie == cookie) {
const char *lastname = last->name;
if (!name && !lastname)
if (name && lastname && !strcmp(lastname, name))
/* the queue overflowed and we already sent the Q_OVERFLOW event */
if (unlikely(dev->event_count > dev->max_events))
/* if the queue overflows, we need to notify user space */
if (unlikely(dev->event_count == dev->max_events))
kevent = kernel_event(-1, IN_Q_OVERFLOW, cookie, NULL);
kevent = kernel_event(watch->wd, mask, cookie, name);
if (unlikely(!kevent))
/* queue the event and wake up anyone waiting */
dev->queue_size += sizeof(struct inotify_event) + kevent->event.len;
list_add_tail(&kevent->list, &dev->events);
* remove_kevent - cleans up and ultimately frees the given kevent
* Caller must hold dev->sem.
static void remove_kevent(struct inotify_device *dev,
struct inotify_kernel_event *kevent)