| inotify | |
| a powerful yet simple file change notification system | |
| Document started 15 Mar 2005 by Robert Love <rml@novell.com> | |
| (i) User Interface | |
| 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: | |
| 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> | |
| 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); | |
| 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. | |
| An existing watch is removed via | |
| int ret = inotify_rm_watch (fd, wd); | |
| Events are provided in the form of an inotify_event structure that is read(2) | |
| 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); | |
| 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. | |
| 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) | |
| 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(). | |
| 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 inotify_handle and each associated inode. | |
| See fs/notify/inotify/inotify_fsnotify.c and fs/notify/inotify/inotify_user.c | |
| for the locking and lifetime rules. | |
| (vi) 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 | |
| unmounted. Watching a file should not require that it be open. | |
| 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, | |
| 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 | |
| should. | |
| 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? | |
| - 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. | |
| 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. | |
| 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 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. | |