X-Git-Url: http://git.rot13.org/?a=blobdiff_plain;f=Documentation%2Ffilesystems%2Finotify.txt;h=59a919f16144df84dc6e3934dcdc22c26611a6ea;hb=cb2ebc59ff52cee770cfd6ba5f23a6cc3c214648;hp=2c716041f57872bca1cd69f1e72f6b847de29fc3;hpb=c514720716c7b109ff980f8b3cb93f9af872c91c;p=powerpc.git diff --git a/Documentation/filesystems/inotify.txt b/Documentation/filesystems/inotify.txt index 2c716041f5..59a919f161 100644 --- 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 + (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 (); +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 @@ -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. -Adding a watch is simple, +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); - } +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 ). 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) -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); -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. -(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 -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? @@ -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 - 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, @@ -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 - 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. @@ -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? - - 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. ;-) @@ -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. + 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. @@ -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 - 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.