Branch data Line data Source code
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 : 5544 : inode_set_iversion_raw(struct inode *inode, u64 val)
94 : : {
95 : 3825 : atomic64_set(&inode->i_version, val);
96 : 0 : }
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 : 19357 : inode_peek_iversion_raw(const struct inode *inode)
111 : : {
112 : 4143 : 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 : 3825 : inode_set_iversion(struct inode *inode, u64 val)
153 : : {
154 : 3825 : 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 : 1719 : inode_set_iversion_queried(struct inode *inode, u64 val)
175 : : {
176 : 1719 : inode_set_iversion_raw(inode, (val << I_VERSION_QUERIED_SHIFT) |
177 : : I_VERSION_QUERIED);
178 : 1719 : }
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 : 2148 : inode_maybe_inc_iversion(struct inode *inode, bool force)
199 : : {
200 : 2148 : 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 : 2148 : smp_mb();
213 : 2148 : cur = inode_peek_iversion_raw(inode);
214 : 2148 : for (;;) {
215 : : /* If flag is clear then we needn't do anything */
216 [ - + - - ]: 2148 : if (!force && !(cur & I_VERSION_QUERIED))
217 : : return false;
218 : :
219 : : /* Since lowest bit is flag, add 2 to avoid it */
220 : 2148 : new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT;
221 : :
222 : 2148 : old = atomic64_cmpxchg(&inode->i_version, cur, new);
223 [ - + ]: 2148 : 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 : 2148 : inode_inc_iversion(struct inode *inode)
240 : : {
241 : 2148 : inode_maybe_inc_iversion(inode, true);
242 : 0 : }
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 : 0 : inode_iversion_need_inc(struct inode *inode)
253 : : {
254 [ # # ]: 0 : 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 : 0 : inode_inc_iversion_raw(struct inode *inode)
270 : : {
271 : 0 : atomic64_inc(&inode->i_version);
272 : 0 : }
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 : 16921 : inode_peek_iversion(const struct inode *inode)
287 : : {
288 [ - + - - ]: 16921 : 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 : 288 : inode_query_iversion(struct inode *inode)
306 : : {
307 : 288 : u64 cur, old, new;
308 : :
309 : 288 : cur = inode_peek_iversion_raw(inode);
310 : 288 : for (;;) {
311 : : /* If flag is already set, then no need to swap */
312 [ + + ]: 288 : 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 : 285 : smp_mb();
319 : 285 : break;
320 : : }
321 : :
322 : 3 : new = cur | I_VERSION_QUERIED;
323 : 3 : old = atomic64_cmpxchg(&inode->i_version, cur, new);
324 [ - + ]: 3 : if (likely(old == cur))
325 : : break;
326 : : cur = old;
327 : : }
328 : 288 : 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 : 0 : inode_eq_iversion_raw(const struct inode *inode, u64 old)
341 : : {
342 : 0 : 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 : 1707 : inode_eq_iversion(const struct inode *inode, u64 old)
358 : : {
359 : 1707 : return inode_peek_iversion(inode) == old;
360 : : }
361 : : #endif
|
