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1 | inotify |
2 | a powerful yet simple file change notification system |
3 | |
4 | |
5 | |
6 | Document started 15 Mar 2005 by Robert Love <rml@novell.com> |
7 | |
8 | |
9 | (i) User Interface |
10 | |
11 | Inotify is controlled by a set of three system calls and normal file I/O on a |
12 | returned file descriptor. |
13 | |
14 | First step in using inotify is to initialise an inotify instance: |
15 | |
16 | int fd = inotify_init (); |
17 | |
18 | Each instance is associated with a unique, ordered queue. |
19 | |
20 | Change events are managed by "watches". A watch is an (object,mask) pair where |
21 | the object is a file or directory and the mask is a bit mask of one or more |
22 | inotify events that the application wishes to receive. See <linux/inotify.h> |
23 | for valid events. A watch is referenced by a watch descriptor, or wd. |
24 | |
25 | Watches are added via a path to the file. |
26 | |
27 | Watches on a directory will return events on any files inside of the directory. |
28 | |
29 | Adding a watch is simple: |
30 | |
31 | int wd = inotify_add_watch (fd, path, mask); |
32 | |
33 | Where "fd" is the return value from inotify_init(), path is the path to the |
34 | object to watch, and mask is the watch mask (see <linux/inotify.h>). |
35 | |
36 | You can update an existing watch in the same manner, by passing in a new mask. |
37 | |
38 | An existing watch is removed via |
39 | |
40 | int ret = inotify_rm_watch (fd, wd); |
41 | |
42 | Events are provided in the form of an inotify_event structure that is read(2) |
43 | from a given inotify instance. The filename is of dynamic length and follows |
44 | the struct. It is of size len. The filename is padded with null bytes to |
45 | ensure proper alignment. This padding is reflected in len. |
46 | |
47 | You can slurp multiple events by passing a large buffer, for example |
48 | |
49 | size_t len = read (fd, buf, BUF_LEN); |
50 | |
51 | Where "buf" is a pointer to an array of "inotify_event" structures at least |
52 | BUF_LEN bytes in size. The above example will return as many events as are |
53 | available and fit in BUF_LEN. |
54 | |
55 | Each inotify instance fd is also select()- and poll()-able. |
56 | |
57 | You can find the size of the current event queue via the standard FIONREAD |
58 | ioctl on the fd returned by inotify_init(). |
59 | |
60 | All watches are destroyed and cleaned up on close. |
61 | |
62 | |
63 | (ii) |
64 | |
65 | Prototypes: |
66 | |
67 | int inotify_init (void); |
68 | int inotify_add_watch (int fd, const char *path, __u32 mask); |
69 | int inotify_rm_watch (int fd, __u32 mask); |
70 | |
71 | |
72 | (iii) Kernel Interface |
73 | |
74 | Inotify's kernel API consists a set of functions for managing watches and an |
75 | event callback. |
76 | |
77 | To use the kernel API, you must first initialize an inotify instance with a set |
78 | of inotify_operations. You are given an opaque inotify_handle, which you use |
79 | for any further calls to inotify. |
80 | |
81 | struct inotify_handle *ih = inotify_init(my_event_handler); |
82 | |
83 | You must provide a function for processing events and a function for destroying |
84 | the inotify watch. |
85 | |
86 | void handle_event(struct inotify_watch *watch, u32 wd, u32 mask, |
87 | u32 cookie, const char *name, struct inode *inode) |
88 | |
89 | watch - the pointer to the inotify_watch that triggered this call |
90 | wd - the watch descriptor |
91 | mask - describes the event that occurred |
92 | cookie - an identifier for synchronizing events |
93 | name - the dentry name for affected files in a directory-based event |
94 | inode - the affected inode in a directory-based event |
95 | |
96 | void destroy_watch(struct inotify_watch *watch) |
97 | |
98 | You may add watches by providing a pre-allocated and initialized inotify_watch |
99 | structure and specifying the inode to watch along with an inotify event mask. |
100 | You must pin the inode during the call. You will likely wish to embed the |
101 | inotify_watch structure in a structure of your own which contains other |
102 | information about the watch. Once you add an inotify watch, it is immediately |
103 | subject to removal depending on filesystem events. You must grab a reference if |
104 | you depend on the watch hanging around after the call. |
105 | |
106 | inotify_init_watch(&my_watch->iwatch); |
107 | inotify_get_watch(&my_watch->iwatch); // optional |
108 | s32 wd = inotify_add_watch(ih, &my_watch->iwatch, inode, mask); |
109 | inotify_put_watch(&my_watch->iwatch); // optional |
110 | |
111 | You may use the watch descriptor (wd) or the address of the inotify_watch for |
112 | other inotify operations. You must not directly read or manipulate data in the |
113 | inotify_watch. Additionally, you must not call inotify_add_watch() more than |
114 | once for a given inotify_watch structure, unless you have first called either |
115 | inotify_rm_watch() or inotify_rm_wd(). |
116 | |
117 | To determine if you have already registered a watch for a given inode, you may |
118 | call inotify_find_watch(), which gives you both the wd and the watch pointer for |
119 | the inotify_watch, or an error if the watch does not exist. |
120 | |
121 | wd = inotify_find_watch(ih, inode, &watchp); |
122 | |
123 | You may use container_of() on the watch pointer to access your own data |
124 | associated with a given watch. When an existing watch is found, |
125 | inotify_find_watch() bumps the refcount before releasing its locks. You must |
126 | put that reference with: |
127 | |
128 | put_inotify_watch(watchp); |
129 | |
130 | Call inotify_find_update_watch() to update the event mask for an existing watch. |
131 | inotify_find_update_watch() returns the wd of the updated watch, or an error if |
132 | the watch does not exist. |
133 | |
134 | wd = inotify_find_update_watch(ih, inode, mask); |
135 | |
136 | An existing watch may be removed by calling either inotify_rm_watch() or |
137 | inotify_rm_wd(). |
138 | |
139 | int ret = inotify_rm_watch(ih, &my_watch->iwatch); |
140 | int ret = inotify_rm_wd(ih, wd); |
141 | |
142 | A watch may be removed while executing your event handler with the following: |
143 | |
144 | inotify_remove_watch_locked(ih, iwatch); |
145 | |
146 | Call inotify_destroy() to remove all watches from your inotify instance and |
147 | release it. If there are no outstanding references, inotify_destroy() will call |
148 | your destroy_watch op for each watch. |
149 | |
150 | inotify_destroy(ih); |
151 | |
152 | When inotify removes a watch, it sends an IN_IGNORED event to your callback. |
153 | You may use this event as an indication to free the watch memory. Note that |
154 | inotify may remove a watch due to filesystem events, as well as by your request. |
155 | If you use IN_ONESHOT, inotify will remove the watch after the first event, at |
156 | which point you may call the final inotify_put_watch. |
157 | |
158 | (iv) Kernel Interface Prototypes |
159 | |
160 | struct inotify_handle *inotify_init(struct inotify_operations *ops); |
161 | |
162 | inotify_init_watch(struct inotify_watch *watch); |
163 | |
164 | s32 inotify_add_watch(struct inotify_handle *ih, |
165 | struct inotify_watch *watch, |
166 | struct inode *inode, u32 mask); |
167 | |
168 | s32 inotify_find_watch(struct inotify_handle *ih, struct inode *inode, |
169 | struct inotify_watch **watchp); |
170 | |
171 | s32 inotify_find_update_watch(struct inotify_handle *ih, |
172 | struct inode *inode, u32 mask); |
173 | |
174 | int inotify_rm_wd(struct inotify_handle *ih, u32 wd); |
175 | |
176 | int inotify_rm_watch(struct inotify_handle *ih, |
177 | struct inotify_watch *watch); |
178 | |
179 | void inotify_remove_watch_locked(struct inotify_handle *ih, |
180 | struct inotify_watch *watch); |
181 | |
182 | void inotify_destroy(struct inotify_handle *ih); |
183 | |
184 | void get_inotify_watch(struct inotify_watch *watch); |
185 | void put_inotify_watch(struct inotify_watch *watch); |
186 | |
187 | |
188 | (v) Internal Kernel Implementation |
189 | |
190 | Each inotify instance is represented by an inotify_handle structure. |
191 | Inotify's userspace consumers also have an inotify_device which is |
192 | associated with the inotify_handle, and on which events are queued. |
193 | |
194 | Each watch is associated with an inotify_watch structure. Watches are chained |
195 | off of each associated inotify_handle and each associated inode. |
196 | |
197 | See fs/inotify.c and fs/inotify_user.c for the locking and lifetime rules. |
198 | |
199 | |
200 | (vi) Rationale |
201 | |
202 | Q: What is the design decision behind not tying the watch to the open fd of |
203 | the watched object? |
204 | |
205 | A: Watches are associated with an open inotify device, not an open file. |
206 | This solves the primary problem with dnotify: keeping the file open pins |
207 | the file and thus, worse, pins the mount. Dnotify is therefore infeasible |
208 | for use on a desktop system with removable media as the media cannot be |
209 | unmounted. Watching a file should not require that it be open. |
210 | |
211 | Q: What is the design decision behind using an-fd-per-instance as opposed to |
212 | an fd-per-watch? |
213 | |
214 | A: An fd-per-watch quickly consumes more file descriptors than are allowed, |
215 | more fd's than are feasible to manage, and more fd's than are optimally |
216 | select()-able. Yes, root can bump the per-process fd limit and yes, users |
217 | can use epoll, but requiring both is a silly and extraneous requirement. |
218 | A watch consumes less memory than an open file, separating the number |
219 | spaces is thus sensible. The current design is what user-space developers |
220 | want: Users initialize inotify, once, and add n watches, requiring but one |
221 | fd and no twiddling with fd limits. Initializing an inotify instance two |
222 | thousand times is silly. If we can implement user-space's preferences |
223 | cleanly--and we can, the idr layer makes stuff like this trivial--then we |
224 | should. |
225 | |
226 | There are other good arguments. With a single fd, there is a single |
227 | item to block on, which is mapped to a single queue of events. The single |
228 | fd returns all watch events and also any potential out-of-band data. If |
229 | every fd was a separate watch, |
230 | |
231 | - There would be no way to get event ordering. Events on file foo and |
232 | file bar would pop poll() on both fd's, but there would be no way to tell |
233 | which happened first. A single queue trivially gives you ordering. Such |
234 | ordering is crucial to existing applications such as Beagle. Imagine |
235 | "mv a b ; mv b a" events without ordering. |
236 | |
237 | - We'd have to maintain n fd's and n internal queues with state, |
238 | versus just one. It is a lot messier in the kernel. A single, linear |
239 | queue is the data structure that makes sense. |
240 | |
241 | - User-space developers prefer the current API. The Beagle guys, for |
242 | example, love it. Trust me, I asked. It is not a surprise: Who'd want |
243 | to manage and block on 1000 fd's via select? |
244 | |
245 | - No way to get out of band data. |
246 | |
247 | - 1024 is still too low. ;-) |
248 | |
249 | When you talk about designing a file change notification system that |
250 | scales to 1000s of directories, juggling 1000s of fd's just does not seem |
251 | the right interface. It is too heavy. |
252 | |
253 | Additionally, it _is_ possible to more than one instance and |
254 | juggle more than one queue and thus more than one associated fd. There |
255 | need not be a one-fd-per-process mapping; it is one-fd-per-queue and a |
256 | process can easily want more than one queue. |
257 | |
258 | Q: Why the system call approach? |
259 | |
260 | A: The poor user-space interface is the second biggest problem with dnotify. |
261 | Signals are a terrible, terrible interface for file notification. Or for |
262 | anything, for that matter. The ideal solution, from all perspectives, is a |
263 | file descriptor-based one that allows basic file I/O and poll/select. |
264 | Obtaining the fd and managing the watches could have been done either via a |
265 | device file or a family of new system calls. We decided to implement a |
266 | family of system calls because that is the preferred approach for new kernel |
267 | interfaces. The only real difference was whether we wanted to use open(2) |
268 | and ioctl(2) or a couple of new system calls. System calls beat ioctls. |
269 | |
270 |
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