Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6

[linux-2.6-omap-h63xx.git] / Documentation / filesystems / inotify.txt

diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt
index 2c716041f57872bca1cd69f1e72f6b847de29fc3..59a919f16144df84dc6e3934dcdc22c26611a6ea 100644 (file)
--- a/Documentation/filesystems/inotify.txt
+++ b/Documentation/filesystems/inotify.txt
@@ -1,18 +1,22 @@
-                                   inotify
-            a powerful yet simple file change notification system
+                                  inotify
+           a powerful yet simple file change notification system

 
 
 
 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
 
 
 
 
 Document started 15 Mar 2005 by Robert Love <rml@novell.com>
 

+

 (i) User Interface
 
 (i) User Interface
 

-Inotify is controlled by a set of three sys calls 
+Inotify is controlled by a set of three system calls and normal file I/O on a
+returned file descriptor.

 
 

-First step in using inotify is to initialise an inotify instance
+First step in using inotify is to initialise an inotify instance:

 
        int fd = inotify_init ();
 
 
        int fd = inotify_init ();
 

+Each instance is associated with a unique, ordered queue.
+

 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>
 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>


@@ -22,51 +26,178 @@ Watches are added via a path to the file.
 
 Watches on a directory will return events on any files inside of the directory.
 
 
 Watches on a directory will return events on any files inside of the directory.
 

-Adding a watch is simple,
+Adding a watch is simple:

 
        int wd = inotify_add_watch (fd, path, mask);
 
 
        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);
-       }
+Where "fd" is the return value from inotify_init(), path is the path to the
+object to watch, and mask is the watch mask (see <linux/inotify.h>).

 
 You can update an existing watch in the same manner, by passing in a new 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
+An existing watch is removed via

 
 

-       inotify_rm_watch (fd, wd);
+       int ret = inotify_rm_watch (fd, wd);

 
 Events are provided in the form of an inotify_event structure that is read(2)
 
 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.
+from a given inotify instance.  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);
 
 
 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.
+Where "buf" is a pointer to an array of "inotify_event" structures at least
+BUF_LEN bytes in size.  The above example will return as many events as are
+available and fit in BUF_LEN.

 
 

-each inotify instance fd is also select()- and poll()-able.
+Each inotify instance fd is also select()- and poll()-able.

 
 

-You can find the size of the current event queue via the FIONREAD ioctl.
+You can find the size of the current event queue via the standard FIONREAD
+ioctl on the fd returned by inotify_init().

 
 All watches are destroyed and cleaned up on close.
 
 
 
 All watches are destroyed and cleaned up on close.
 
 

-(ii) Internal Kernel Implementation
+(ii)
+
+Prototypes:
+
+       int inotify_init (void);
+       int inotify_add_watch (int fd, const char *path, __u32 mask);
+       int inotify_rm_watch (int fd, __u32 mask);
+
+
+(iii) Kernel Interface
+
+Inotify's kernel API consists a set of functions for managing watches and an
+event callback.
+
+To use the kernel API, you must first initialize an inotify instance with a set
+of inotify_operations.  You are given an opaque inotify_handle, which you use
+for any further calls to inotify.
+
+    struct inotify_handle *ih = inotify_init(my_event_handler);
+
+You must provide a function for processing events and a function for destroying
+the inotify watch.
+
+    void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
+                     u32 cookie, const char *name, struct inode *inode)
+
+       watch - the pointer to the inotify_watch that triggered this call
+       wd - the watch descriptor
+       mask - describes the event that occurred
+       cookie - an identifier for synchronizing events
+       name - the dentry name for affected files in a directory-based event
+       inode - the affected inode in a directory-based event
+
+    void destroy_watch(struct inotify_watch *watch)
+
+You may add watches by providing a pre-allocated and initialized inotify_watch
+structure and specifying the inode to watch along with an inotify event mask.
+You must pin the inode during the call.  You will likely wish to embed the
+inotify_watch structure in a structure of your own which contains other
+information about the watch.  Once you add an inotify watch, it is immediately
+subject to removal depending on filesystem events.  You must grab a reference if
+you depend on the watch hanging around after the call.
+
+    inotify_init_watch(&my_watch->iwatch);
+    inotify_get_watch(&my_watch->iwatch);      // optional
+    s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask);
+    inotify_put_watch(&my_watch->iwatch);      // optional
+
+You may use the watch descriptor (wd) or the address of the inotify_watch for
+other inotify operations.  You must not directly read or manipulate data in the
+inotify_watch.  Additionally, you must not call inotify_add_watch() more than
+once for a given inotify_watch structure, unless you have first called either
+inotify_rm_watch() or inotify_rm_wd().
+
+To determine if you have already registered a watch for a given inode, you may
+call inotify_find_watch(), which gives you both the wd and the watch pointer for
+the inotify_watch, or an error if the watch does not exist.
+
+    wd = inotify_find_watch(ih, inode, &watchp);
+
+You may use container_of() on the watch pointer to access your own data
+associated with a given watch.  When an existing watch is found,
+inotify_find_watch() bumps the refcount before releasing its locks.  You must
+put that reference with:
+
+    put_inotify_watch(watchp);
+
+Call inotify_find_update_watch() to update the event mask for an existing watch.
+inotify_find_update_watch() returns the wd of the updated watch, or an error if
+the watch does not exist.
+
+    wd = inotify_find_update_watch(ih, inode, mask);
+
+An existing watch may be removed by calling either inotify_rm_watch() or
+inotify_rm_wd().

 
 

-Each open inotify instance is associated with an inotify_device structure.
+    int ret = inotify_rm_watch(ih, &my_watch->iwatch);
+    int ret = inotify_rm_wd(ih, wd);
+
+A watch may be removed while executing your event handler with the following:
+
+    inotify_remove_watch_locked(ih, iwatch);
+
+Call inotify_destroy() to remove all watches from your inotify instance and
+release it.  If there are no outstanding references, inotify_destroy() will call
+your destroy_watch op for each watch.
+
+    inotify_destroy(ih);
+
+When inotify removes a watch, it sends an IN_IGNORED event to your callback.
+You may use this event as an indication to free the watch memory.  Note that
+inotify may remove a watch due to filesystem events, as well as by your request.
+If you use IN_ONESHOT, inotify will remove the watch after the first event, at
+which point you may call the final inotify_put_watch.
+
+(iv) Kernel Interface Prototypes
+
+       struct inotify_handle *inotify_init(struct inotify_operations *ops);
+
+       inotify_init_watch(struct inotify_watch *watch);
+
+       s32 inotify_add_watch(struct inotify_handle *ih,
+                             struct inotify_watch *watch,
+                             struct inode *inode, u32 mask);
+
+       s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
+                              struct inotify_watch **watchp);
+
+       s32 inotify_find_update_watch(struct inotify_handle *ih,
+                                     struct inode *inode, u32 mask);
+
+       int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
+
+       int inotify_rm_watch(struct inotify_handle *ih,
+                            struct inotify_watch *watch);
+
+       void inotify_remove_watch_locked(struct inotify_handle *ih,
+                                        struct inotify_watch *watch);
+
+       void inotify_destroy(struct inotify_handle *ih);
+
+       void get_inotify_watch(struct inotify_watch *watch);
+       void put_inotify_watch(struct inotify_watch *watch);
+
+
+(v) Internal Kernel Implementation
+
+Each inotify instance is represented by an inotify_handle structure.
+Inotify's userspace consumers also have an inotify_device which is
+associated with the inotify_handle, and on which events are queued.

 
 Each watch is associated with an inotify_watch structure.  Watches are chained
 
 Each watch is associated with an inotify_watch structure.  Watches are chained

-off of each associated device and each associated inode.
+off of each associated inotify_handle and each associated inode.

 
 

-See fs/inotify.c for the locking and lifetime rules.
+See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules.

 
 
 
 

-(iii) Rationale
+(vi) Rationale

 
 Q: What is the design decision behind not tying the watch to the open fd of
    the watched object?
 
 Q: What is the design decision behind not tying the watch to the open fd of
    the watched object?


@@ -75,9 +206,9 @@ 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
    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

-   unmounted.
+   unmounted.  Watching a file should not require that it be open.

 
 

-Q: What is the design decision behind using an-fd-per-device as opposed to
+Q: What is the design decision behind using an-fd-per-instance as opposed to

    an fd-per-watch?
 
 A: An fd-per-watch quickly consumes more file descriptors than are allowed,
    an fd-per-watch?
 
 A: An fd-per-watch quickly consumes more file descriptors than are allowed,


@@ -86,8 +217,8 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed,
    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
    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
+   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 
    should.
    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 
    should.


@@ -111,9 +242,6 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed,
      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?
 
      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.  ;-)
    - No way to get out of band data.
 
    - 1024 is still too low.  ;-)


@@ -122,6 +250,11 @@ A: An fd-per-watch quickly consumes more file descriptors than are allowed,
    scales to 1000s of directories, juggling 1000s of fd's just does not seem
    the right interface.  It is too heavy.
 
    scales to 1000s of directories, juggling 1000s of fd's just does not seem
    the right interface.  It is too heavy.
 

+   Additionally, it _is_ possible to  more than one instance  and
+   juggle more than one queue and thus more than one associated fd.  There
+   need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
+   process can easily want more than one queue.
+

 Q: Why the system call approach?
 
 A: The poor user-space interface is the second biggest problem with dnotify.
 Q: Why the system call approach?
 
 A: The poor user-space interface is the second biggest problem with dnotify.


@@ -130,9 +263,7 @@ A: The poor user-space interface is the second biggest problem with dnotify.
    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
    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.
+   family of system calls because that is the preferred approach for new kernel
+   interfaces.  The only real difference was whether we wanted to use open(2)
+   and ioctl(2) or a couple of new system calls.  System calls beat ioctls.