270 lines
11 KiB
Plaintext
270 lines
11 KiB
Plaintext
inotify
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a powerful yet simple file change notification system
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Document started 15 Mar 2005 by Robert Love <rml@novell.com>
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(i) User Interface
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Inotify is controlled by a set of three system calls and normal file I/O on a
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returned file descriptor.
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First step in using inotify is to initialise an inotify instance:
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int fd = inotify_init ();
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Each instance is associated with a unique, ordered queue.
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Change events are managed by "watches". A watch is an (object,mask) pair where
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the object is a file or directory and the mask is a bit mask of one or more
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inotify events that the application wishes to receive. See <linux/inotify.h>
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for valid events. A watch is referenced by a watch descriptor, or wd.
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Watches are added via a path to the file.
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Watches on a directory will return events on any files inside of the directory.
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Adding a watch is simple:
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int wd = inotify_add_watch (fd, path, mask);
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Where "fd" is the return value from inotify_init(), path is the path to the
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object to watch, and mask is the watch mask (see <linux/inotify.h>).
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You can update an existing watch in the same manner, by passing in a new mask.
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An existing watch is removed via
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int ret = inotify_rm_watch (fd, wd);
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Events are provided in the form of an inotify_event structure that is read(2)
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from a given inotify instance. The filename is of dynamic length and follows
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the struct. It is of size len. The filename is padded with null bytes to
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ensure proper alignment. This padding is reflected in len.
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You can slurp multiple events by passing a large buffer, for example
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size_t len = read (fd, buf, BUF_LEN);
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Where "buf" is a pointer to an array of "inotify_event" structures at least
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BUF_LEN bytes in size. The above example will return as many events as are
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available and fit in BUF_LEN.
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Each inotify instance fd is also select()- and poll()-able.
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You can find the size of the current event queue via the standard FIONREAD
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ioctl on the fd returned by inotify_init().
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All watches are destroyed and cleaned up on close.
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(ii)
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Prototypes:
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int inotify_init (void);
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int inotify_add_watch (int fd, const char *path, __u32 mask);
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int inotify_rm_watch (int fd, __u32 mask);
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(iii) Kernel Interface
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Inotify's kernel API consists a set of functions for managing watches and an
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event callback.
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To use the kernel API, you must first initialize an inotify instance with a set
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of inotify_operations. You are given an opaque inotify_handle, which you use
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for any further calls to inotify.
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struct inotify_handle *ih = inotify_init(my_event_handler);
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You must provide a function for processing events and a function for destroying
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the inotify watch.
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void handle_event(struct inotify_watch *watch, u32 wd, u32 mask,
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u32 cookie, const char *name, struct inode *inode)
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watch - the pointer to the inotify_watch that triggered this call
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wd - the watch descriptor
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mask - describes the event that occurred
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cookie - an identifier for synchronizing events
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name - the dentry name for affected files in a directory-based event
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inode - the affected inode in a directory-based event
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void destroy_watch(struct inotify_watch *watch)
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You may add watches by providing a pre-allocated and initialized inotify_watch
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structure and specifying the inode to watch along with an inotify event mask.
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You must pin the inode during the call. You will likely wish to embed the
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inotify_watch structure in a structure of your own which contains other
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information about the watch. Once you add an inotify watch, it is immediately
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subject to removal depending on filesystem events. You must grab a reference if
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you depend on the watch hanging around after the call.
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inotify_init_watch(&my_watch->iwatch);
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inotify_get_watch(&my_watch->iwatch); // optional
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s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask);
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inotify_put_watch(&my_watch->iwatch); // optional
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You may use the watch descriptor (wd) or the address of the inotify_watch for
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other inotify operations. You must not directly read or manipulate data in the
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inotify_watch. Additionally, you must not call inotify_add_watch() more than
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once for a given inotify_watch structure, unless you have first called either
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inotify_rm_watch() or inotify_rm_wd().
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To determine if you have already registered a watch for a given inode, you may
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call inotify_find_watch(), which gives you both the wd and the watch pointer for
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the inotify_watch, or an error if the watch does not exist.
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wd = inotify_find_watch(ih, inode, &watchp);
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You may use container_of() on the watch pointer to access your own data
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associated with a given watch. When an existing watch is found,
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inotify_find_watch() bumps the refcount before releasing its locks. You must
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put that reference with:
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put_inotify_watch(watchp);
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Call inotify_find_update_watch() to update the event mask for an existing watch.
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inotify_find_update_watch() returns the wd of the updated watch, or an error if
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the watch does not exist.
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wd = inotify_find_update_watch(ih, inode, mask);
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An existing watch may be removed by calling either inotify_rm_watch() or
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inotify_rm_wd().
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int ret = inotify_rm_watch(ih, &my_watch->iwatch);
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int ret = inotify_rm_wd(ih, wd);
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A watch may be removed while executing your event handler with the following:
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inotify_remove_watch_locked(ih, iwatch);
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Call inotify_destroy() to remove all watches from your inotify instance and
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release it. If there are no outstanding references, inotify_destroy() will call
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your destroy_watch op for each watch.
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inotify_destroy(ih);
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When inotify removes a watch, it sends an IN_IGNORED event to your callback.
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You may use this event as an indication to free the watch memory. Note that
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inotify may remove a watch due to filesystem events, as well as by your request.
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If you use IN_ONESHOT, inotify will remove the watch after the first event, at
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which point you may call the final inotify_put_watch.
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(iv) Kernel Interface Prototypes
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struct inotify_handle *inotify_init(struct inotify_operations *ops);
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inotify_init_watch(struct inotify_watch *watch);
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s32 inotify_add_watch(struct inotify_handle *ih,
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struct inotify_watch *watch,
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struct inode *inode, u32 mask);
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s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode,
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struct inotify_watch **watchp);
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s32 inotify_find_update_watch(struct inotify_handle *ih,
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struct inode *inode, u32 mask);
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int inotify_rm_wd(struct inotify_handle *ih, u32 wd);
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int inotify_rm_watch(struct inotify_handle *ih,
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struct inotify_watch *watch);
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void inotify_remove_watch_locked(struct inotify_handle *ih,
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struct inotify_watch *watch);
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void inotify_destroy(struct inotify_handle *ih);
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void get_inotify_watch(struct inotify_watch *watch);
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void put_inotify_watch(struct inotify_watch *watch);
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(v) Internal Kernel Implementation
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Each inotify instance is represented by an inotify_handle structure.
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Inotify's userspace consumers also have an inotify_device which is
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associated with the inotify_handle, and on which events are queued.
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Each watch is associated with an inotify_watch structure. Watches are chained
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off of each associated inotify_handle and each associated inode.
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See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules.
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(vi) Rationale
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Q: What is the design decision behind not tying the watch to the open fd of
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the watched object?
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A: Watches are associated with an open inotify device, not an open file.
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This solves the primary problem with dnotify: keeping the file open pins
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the file and thus, worse, pins the mount. Dnotify is therefore infeasible
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for use on a desktop system with removable media as the media cannot be
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unmounted. Watching a file should not require that it be open.
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Q: What is the design decision behind using an-fd-per-instance as opposed to
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an fd-per-watch?
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A: An fd-per-watch quickly consumes more file descriptors than are allowed,
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more fd's than are feasible to manage, and more fd's than are optimally
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select()-able. Yes, root can bump the per-process fd limit and yes, users
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can use epoll, but requiring both is a silly and extraneous requirement.
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A watch consumes less memory than an open file, separating the number
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spaces is thus sensible. The current design is what user-space developers
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want: Users initialize inotify, once, and add n watches, requiring but one
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fd and no twiddling with fd limits. Initializing an inotify instance two
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thousand times is silly. If we can implement user-space's preferences
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cleanly--and we can, the idr layer makes stuff like this trivial--then we
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should.
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There are other good arguments. With a single fd, there is a single
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item to block on, which is mapped to a single queue of events. The single
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fd returns all watch events and also any potential out-of-band data. If
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every fd was a separate watch,
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- There would be no way to get event ordering. Events on file foo and
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file bar would pop poll() on both fd's, but there would be no way to tell
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which happened first. A single queue trivially gives you ordering. Such
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ordering is crucial to existing applications such as Beagle. Imagine
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"mv a b ; mv b a" events without ordering.
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- We'd have to maintain n fd's and n internal queues with state,
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versus just one. It is a lot messier in the kernel. A single, linear
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queue is the data structure that makes sense.
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- User-space developers prefer the current API. The Beagle guys, for
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example, love it. Trust me, I asked. It is not a surprise: Who'd want
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to manage and block on 1000 fd's via select?
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- No way to get out of band data.
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- 1024 is still too low. ;-)
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When you talk about designing a file change notification system that
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scales to 1000s of directories, juggling 1000s of fd's just does not seem
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the right interface. It is too heavy.
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Additionally, it _is_ possible to more than one instance and
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juggle more than one queue and thus more than one associated fd. There
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need not be a one-fd-per-process mapping; it is one-fd-per-queue and a
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process can easily want more than one queue.
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Q: Why the system call approach?
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A: The poor user-space interface is the second biggest problem with dnotify.
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Signals are a terrible, terrible interface for file notification. Or for
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anything, for that matter. The ideal solution, from all perspectives, is a
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file descriptor-based one that allows basic file I/O and poll/select.
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Obtaining the fd and managing the watches could have been done either via a
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device file or a family of new system calls. We decided to implement a
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family of system calls because that is the preferred approach for new kernel
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interfaces. The only real difference was whether we wanted to use open(2)
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and ioctl(2) or a couple of new system calls. System calls beat ioctls.
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