1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_IVERSION_H
3 #define _LINUX_IVERSION_H
4
5 #include <linux/fs.h>
6
7 /*
8 * The inode->i_version field:
9 * ---------------------------
10 * The change attribute (i_version) is mandated by NFSv4 and is mostly for
11 * knfsd, but is also used for other purposes (e.g. IMA). The i_version must
12 * appear different to observers if there was a change to the inode's data or
13 * metadata since it was last queried.
14 *
15 * Observers see the i_version as a 64-bit number that never decreases. If it
16 * remains the same since it was last checked, then nothing has changed in the
17 * inode. If it's different then something has changed. Observers cannot infer
18 * anything about the nature or magnitude of the changes from the value, only
19 * that the inode has changed in some fashion.
20 *
21 * Not all filesystems properly implement the i_version counter. Subsystems that
22 * want to use i_version field on an inode should first check whether the
23 * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION macro).
24 *
25 * Those that set SB_I_VERSION will automatically have their i_version counter
26 * incremented on writes to normal files. If the SB_I_VERSION is not set, then
27 * the VFS will not touch it on writes, and the filesystem can use it how it
28 * wishes. Note that the filesystem is always responsible for updating the
29 * i_version on namespace changes in directories (mkdir, rmdir, unlink, etc.).
30 * We consider these sorts of filesystems to have a kernel-managed i_version.
31 *
32 * It may be impractical for filesystems to keep i_version updates atomic with
33 * respect to the changes that cause them. They should, however, guarantee
34 * that i_version updates are never visible before the changes that caused
35 * them. Also, i_version updates should never be delayed longer than it takes
36 * the original change to reach disk.
37 *
38 * This implementation uses the low bit in the i_version field as a flag to
39 * track when the value has been queried. If it has not been queried since it
40 * was last incremented, we can skip the increment in most cases.
41 *
42 * In the event that we're updating the ctime, we will usually go ahead and
43 * bump the i_version anyway. Since that has to go to stable storage in some
44 * fashion, we might as well increment it as well.
45 *
46 * With this implementation, the value should always appear to observers to
47 * increase over time if the file has changed. It's recommended to use
48 * inode_eq_iversion() helper to compare values.
49 *
50 * Note that some filesystems (e.g. NFS and AFS) just use the field to store
51 * a server-provided value (for the most part). For that reason, those
52 * filesystems do not set SB_I_VERSION. These filesystems are considered to
53 * have a self-managed i_version.
54 *
55 * Persistently storing the i_version
56 * ----------------------------------
57 * Queries of the i_version field are not gated on them hitting the backing
58 * store. It's always possible that the host could crash after allowing
59 * a query of the value but before it has made it to disk.
60 *
61 * To mitigate this problem, filesystems should always use
62 * inode_set_iversion_queried when loading an existing inode from disk. This
63 * ensures that the next attempted inode increment will result in the value
64 * changing.
65 *
66 * Storing the value to disk therefore does not count as a query, so those
67 * filesystems should use inode_peek_iversion to grab the value to be stored.
68 * There is no need to flag the value as having been queried in that case.
69 */
70
71 /*
72 * We borrow the lowest bit in the i_version to use as a flag to tell whether
73 * it has been queried since we last incremented it. If it has, then we must
74 * increment it on the next change. After that, we can clear the flag and
75 * avoid incrementing it again until it has again been queried.
76 */
77 #define I_VERSION_QUERIED_SHIFT (1)
78 #define I_VERSION_QUERIED (1ULL << (I_VERSION_QUERIED_SHIFT - 1))
79 #define I_VERSION_INCREMENT (1ULL << I_VERSION_QUERIED_SHIFT)
80
81 /**
82 * inode_set_iversion_raw - set i_version to the specified raw value
83 * @inode: inode to set
84 * @val: new i_version value to set
85 *
86 * Set @inode's i_version field to @val. This function is for use by
87 * filesystems that self-manage the i_version.
88 *
89 * For example, the NFS client stores its NFSv4 change attribute in this way,
90 * and the AFS client stores the data_version from the server here.
91 */
92 static inline void
93 inode_set_iversion_raw(struct inode *inode, u64 val)
94 {
95 atomic64_set(&inode->i_version, val);
96 }
97
98 /**
99 * inode_peek_iversion_raw - grab a "raw" iversion value
100 * @inode: inode from which i_version should be read
101 *
102 * Grab a "raw" inode->i_version value and return it. The i_version is not
103 * flagged or converted in any way. This is mostly used to access a self-managed
104 * i_version.
105 *
106 * With those filesystems, we want to treat the i_version as an entirely
107 * opaque value.
108 */
109 static inline u64
110 inode_peek_iversion_raw(const struct inode *inode)
111 {
112 return atomic64_read(&inode->i_version);
113 }
114
115 /**
116 * inode_set_max_iversion_raw - update i_version new value is larger
117 * @inode: inode to set
118 * @val: new i_version to set
119 *
120 * Some self-managed filesystems (e.g Ceph) will only update the i_version
121 * value if the new value is larger than the one we already have.
122 */
123 static inline void
124 inode_set_max_iversion_raw(struct inode *inode, u64 val)
125 {
126 u64 cur, old;
127
128 cur = inode_peek_iversion_raw(inode);
129 for (;;) {
130 if (cur > val)
131 break;
132 old = atomic64_cmpxchg(&inode->i_version, cur, val);
133 if (likely(old == cur))
134 break;
135 cur = old;
136 }
137 }
138
139 /**
140 * inode_set_iversion - set i_version to a particular value
141 * @inode: inode to set
142 * @val: new i_version value to set
143 *
144 * Set @inode's i_version field to @val. This function is for filesystems with
145 * a kernel-managed i_version, for initializing a newly-created inode from
146 * scratch.
147 *
148 * In this case, we do not set the QUERIED flag since we know that this value
149 * has never been queried.
150 */
151 static inline void
152 inode_set_iversion(struct inode *inode, u64 val)
153 {
154 inode_set_iversion_raw(inode, val << I_VERSION_QUERIED_SHIFT);
155 }
156
157 /**
158 * inode_set_iversion_queried - set i_version to a particular value as quereied
159 * @inode: inode to set
160 * @val: new i_version value to set
161 *
162 * Set @inode's i_version field to @val, and flag it for increment on the next
163 * change.
164 *
165 * Filesystems that persistently store the i_version on disk should use this
166 * when loading an existing inode from disk.
167 *
168 * When loading in an i_version value from a backing store, we can't be certain
169 * that it wasn't previously viewed before being stored. Thus, we must assume
170 * that it was, to ensure that we don't end up handing out the same value for
171 * different versions of the same inode.
172 */
173 static inline void
174 inode_set_iversion_queried(struct inode *inode, u64 val)
175 {
176 inode_set_iversion_raw(inode, (val << I_VERSION_QUERIED_SHIFT) |
177 I_VERSION_QUERIED);
178 }
179
180 /**
181 * inode_maybe_inc_iversion - increments i_version
182 * @inode: inode with the i_version that should be updated
183 * @force: increment the counter even if it's not necessary?
184 *
185 * Every time the inode is modified, the i_version field must be seen to have
186 * changed by any observer.
187 *
188 * If "force" is set or the QUERIED flag is set, then ensure that we increment
189 * the value, and clear the queried flag.
190 *
191 * In the common case where neither is set, then we can return "false" without
192 * updating i_version.
193 *
194 * If this function returns false, and no other metadata has changed, then we
195 * can avoid logging the metadata.
196 */
197 static inline bool
198 inode_maybe_inc_iversion(struct inode *inode, bool force)
199 {
200 u64 cur, old, new;
201
202 /*
203 * The i_version field is not strictly ordered with any other inode
204 * information, but the legacy inode_inc_iversion code used a spinlock
205 * to serialize increments.
206 *
207 * Here, we add full memory barriers to ensure that any de-facto
208 * ordering with other info is preserved.
209 *
210 * This barrier pairs with the barrier in inode_query_iversion()
211 */
212 smp_mb();
213 cur = inode_peek_iversion_raw(inode);
214 for (;;) {
215 /* If flag is clear then we needn't do anything */
216 if (!force && !(cur & I_VERSION_QUERIED))
217 return false;
218
219 /* Since lowest bit is flag, add 2 to avoid it */
220 new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
221
222 old = atomic64_cmpxchg(&inode->i_version, cur, new);
223 if (likely(old == cur))
224 break;
225 cur = old;
226 }
227 return true;
228 }
229
230
231 /**
232 * inode_inc_iversion - forcibly increment i_version
233 * @inode: inode that needs to be updated
234 *
235 * Forcbily increment the i_version field. This always results in a change to
236 * the observable value.
237 */
238 static inline void
239 inode_inc_iversion(struct inode *inode)
240 {
241 inode_maybe_inc_iversion(inode, true);
242 }
243
244 /**
245 * inode_iversion_need_inc - is the i_version in need of being incremented?
246 * @inode: inode to check
247 *
248 * Returns whether the inode->i_version counter needs incrementing on the next
249 * change. Just fetch the value and check the QUERIED flag.
250 */
251 static inline bool
252 inode_iversion_need_inc(struct inode *inode)
253 {
254 return inode_peek_iversion_raw(inode) & I_VERSION_QUERIED;
255 }
256
257 /**
258 * inode_inc_iversion_raw - forcibly increment raw i_version
259 * @inode: inode that needs to be updated
260 *
261 * Forcbily increment the raw i_version field. This always results in a change
262 * to the raw value.
263 *
264 * NFS will use the i_version field to store the value from the server. It
265 * mostly treats it as opaque, but in the case where it holds a write
266 * delegation, it must increment the value itself. This function does that.
267 */
268 static inline void
269 inode_inc_iversion_raw(struct inode *inode)
270 {
271 atomic64_inc(&inode->i_version);
272 }
273
274 /**
275 * inode_peek_iversion - read i_version without flagging it to be incremented
276 * @inode: inode from which i_version should be read
277 *
278 * Read the inode i_version counter for an inode without registering it as a
279 * query.
280 *
281 * This is typically used by local filesystems that need to store an i_version
282 * on disk. In that situation, it's not necessary to flag it as having been
283 * viewed, as the result won't be used to gauge changes from that point.
284 */
285 static inline u64
286 inode_peek_iversion(const struct inode *inode)
287 {
288 return inode_peek_iversion_raw(inode) >> I_VERSION_QUERIED_SHIFT;
289 }
290
291 /**
292 * inode_query_iversion - read i_version for later use
293 * @inode: inode from which i_version should be read
294 *
295 * Read the inode i_version counter. This should be used by callers that wish
296 * to store the returned i_version for later comparison. This will guarantee
297 * that a later query of the i_version will result in a different value if
298 * anything has changed.
299 *
300 * In this implementation, we fetch the current value, set the QUERIED flag and
301 * then try to swap it into place with a cmpxchg, if it wasn't already set. If
302 * that fails, we try again with the newly fetched value from the cmpxchg.
303 */
304 static inline u64
305 inode_query_iversion(struct inode *inode)
306 {
307 u64 cur, old, new;
308
309 cur = inode_peek_iversion_raw(inode);
310 for (;;) {
311 /* If flag is already set, then no need to swap */
312 if (cur & I_VERSION_QUERIED) {
313 /*
314 * This barrier (and the implicit barrier in the
315 * cmpxchg below) pairs with the barrier in
316 * inode_maybe_inc_iversion().
317 */
318 smp_mb();
319 break;
320 }
321
322 new = cur | I_VERSION_QUERIED;
323 old = atomic64_cmpxchg(&inode->i_version, cur, new);
324 if (likely(old == cur))
325 break;
326 cur = old;
327 }
328 return cur >> I_VERSION_QUERIED_SHIFT;
329 }
330
331 /**
332 * inode_eq_iversion_raw - check whether the raw i_version counter has changed
333 * @inode: inode to check
334 * @old: old value to check against its i_version
335 *
336 * Compare the current raw i_version counter with a previous one. Returns true
337 * if they are the same or false if they are different.
338 */
339 static inline bool
340 inode_eq_iversion_raw(const struct inode *inode, u64 old)
341 {
342 return inode_peek_iversion_raw(inode) == old;
343 }
344
345 /**
346 * inode_eq_iversion - check whether the i_version counter has changed
347 * @inode: inode to check
348 * @old: old value to check against its i_version
349 *
350 * Compare an i_version counter with a previous one. Returns true if they are
351 * the same, and false if they are different.
352 *
353 * Note that we don't need to set the QUERIED flag in this case, as the value
354 * in the inode is not being recorded for later use.
355 */
356 static inline bool
357 inode_eq_iversion(const struct inode *inode, u64 old)
358 {
359 return inode_peek_iversion(inode) == old;
360 }
361 #endif