diff options
Diffstat (limited to 'include/linux/iversion.h')
| -rw-r--r-- | include/linux/iversion.h | 337 |
1 files changed, 337 insertions, 0 deletions
diff --git a/include/linux/iversion.h b/include/linux/iversion.h new file mode 100644 index 000000000000..3d2fd06495ec --- /dev/null +++ b/include/linux/iversion.h @@ -0,0 +1,337 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _LINUX_IVERSION_H +#define _LINUX_IVERSION_H + +#include <linux/fs.h> + +/* + * The inode->i_version field: + * --------------------------- + * The change attribute (i_version) is mandated by NFSv4 and is mostly for + * knfsd, but is also used for other purposes (e.g. IMA). The i_version must + * appear different to observers if there was a change to the inode's data or + * metadata since it was last queried. + * + * Observers see the i_version as a 64-bit number that never decreases. If it + * remains the same since it was last checked, then nothing has changed in the + * inode. If it's different then something has changed. Observers cannot infer + * anything about the nature or magnitude of the changes from the value, only + * that the inode has changed in some fashion. + * + * Not all filesystems properly implement the i_version counter. Subsystems that + * want to use i_version field on an inode should first check whether the + * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION macro). + * + * Those that set SB_I_VERSION will automatically have their i_version counter + * incremented on writes to normal files. If the SB_I_VERSION is not set, then + * the VFS will not touch it on writes, and the filesystem can use it how it + * wishes. Note that the filesystem is always responsible for updating the + * i_version on namespace changes in directories (mkdir, rmdir, unlink, etc.). + * We consider these sorts of filesystems to have a kernel-managed i_version. + * + * It may be impractical for filesystems to keep i_version updates atomic with + * respect to the changes that cause them. They should, however, guarantee + * that i_version updates are never visible before the changes that caused + * them. Also, i_version updates should never be delayed longer than it takes + * the original change to reach disk. + * + * This implementation uses the low bit in the i_version field as a flag to + * track when the value has been queried. If it has not been queried since it + * was last incremented, we can skip the increment in most cases. + * + * In the event that we're updating the ctime, we will usually go ahead and + * bump the i_version anyway. Since that has to go to stable storage in some + * fashion, we might as well increment it as well. + * + * With this implementation, the value should always appear to observers to + * increase over time if the file has changed. It's recommended to use + * inode_cmp_iversion() helper to compare values. + * + * Note that some filesystems (e.g. NFS and AFS) just use the field to store + * a server-provided value (for the most part). For that reason, those + * filesystems do not set SB_I_VERSION. These filesystems are considered to + * have a self-managed i_version. + * + * Persistently storing the i_version + * ---------------------------------- + * Queries of the i_version field are not gated on them hitting the backing + * store. It's always possible that the host could crash after allowing + * a query of the value but before it has made it to disk. + * + * To mitigate this problem, filesystems should always use + * inode_set_iversion_queried when loading an existing inode from disk. This + * ensures that the next attempted inode increment will result in the value + * changing. + * + * Storing the value to disk therefore does not count as a query, so those + * filesystems should use inode_peek_iversion to grab the value to be stored. + * There is no need to flag the value as having been queried in that case. + */ + +/* + * We borrow the lowest bit in the i_version to use as a flag to tell whether + * it has been queried since we last incremented it. If it has, then we must + * increment it on the next change. After that, we can clear the flag and + * avoid incrementing it again until it has again been queried. + */ +#define I_VERSION_QUERIED_SHIFT (1) +#define I_VERSION_QUERIED (1ULL << (I_VERSION_QUERIED_SHIFT - 1)) +#define I_VERSION_INCREMENT (1ULL << I_VERSION_QUERIED_SHIFT) + +/** + * inode_set_iversion_raw - set i_version to the specified raw value + * @inode: inode to set + * @val: new i_version value to set + * + * Set @inode's i_version field to @val. This function is for use by + * filesystems that self-manage the i_version. + * + * For example, the NFS client stores its NFSv4 change attribute in this way, + * and the AFS client stores the data_version from the server here. + */ +static inline void +inode_set_iversion_raw(struct inode *inode, u64 val) +{ + atomic64_set(&inode->i_version, val); +} + +/** + * inode_peek_iversion_raw - grab a "raw" iversion value + * @inode: inode from which i_version should be read + * + * Grab a "raw" inode->i_version value and return it. The i_version is not + * flagged or converted in any way. This is mostly used to access a self-managed + * i_version. + * + * With those filesystems, we want to treat the i_version as an entirely + * opaque value. + */ +static inline u64 +inode_peek_iversion_raw(const struct inode *inode) +{ + return atomic64_read(&inode->i_version); +} + +/** + * inode_set_iversion - set i_version to a particular value + * @inode: inode to set + * @val: new i_version value to set + * + * Set @inode's i_version field to @val. This function is for filesystems with + * a kernel-managed i_version, for initializing a newly-created inode from + * scratch. + * + * In this case, we do not set the QUERIED flag since we know that this value + * has never been queried. + */ +static inline void +inode_set_iversion(struct inode *inode, u64 val) +{ + inode_set_iversion_raw(inode, val << I_VERSION_QUERIED_SHIFT); +} + +/** + * inode_set_iversion_queried - set i_version to a particular value as quereied + * @inode: inode to set + * @val: new i_version value to set + * + * Set @inode's i_version field to @val, and flag it for increment on the next + * change. + * + * Filesystems that persistently store the i_version on disk should use this + * when loading an existing inode from disk. + * + * When loading in an i_version value from a backing store, we can't be certain + * that it wasn't previously viewed before being stored. Thus, we must assume + * that it was, to ensure that we don't end up handing out the same value for + * different versions of the same inode. + */ +static inline void +inode_set_iversion_queried(struct inode *inode, u64 val) +{ + inode_set_iversion_raw(inode, (val << I_VERSION_QUERIED_SHIFT) | + I_VERSION_QUERIED); +} + +/** + * inode_maybe_inc_iversion - increments i_version + * @inode: inode with the i_version that should be updated + * @force: increment the counter even if it's not necessary? + * + * Every time the inode is modified, the i_version field must be seen to have + * changed by any observer. + * + * If "force" is set or the QUERIED flag is set, then ensure that we increment + * the value, and clear the queried flag. + * + * In the common case where neither is set, then we can return "false" without + * updating i_version. + * + * If this function returns false, and no other metadata has changed, then we + * can avoid logging the metadata. + */ +static inline bool +inode_maybe_inc_iversion(struct inode *inode, bool force) +{ + u64 cur, old, new; + + /* + * The i_version field is not strictly ordered with any other inode + * information, but the legacy inode_inc_iversion code used a spinlock + * to serialize increments. + * + * Here, we add full memory barriers to ensure that any de-facto + * ordering with other info is preserved. + * + * This barrier pairs with the barrier in inode_query_iversion() + */ + smp_mb(); + cur = inode_peek_iversion_raw(inode); + for (;;) { + /* If flag is clear then we needn't do anything */ + if (!force && !(cur & I_VERSION_QUERIED)) + return false; + + /* Since lowest bit is flag, add 2 to avoid it */ + new = (cur & ~I_VERSION_QUERIED) + I_VERSION_INCREMENT; + + old = atomic64_cmpxchg(&inode->i_version, cur, new); + if (likely(old == cur)) + break; + cur = old; + } + return true; +} + + +/** + * inode_inc_iversion - forcibly increment i_version + * @inode: inode that needs to be updated + * + * Forcbily increment the i_version field. This always results in a change to + * the observable value. + */ +static inline void +inode_inc_iversion(struct inode *inode) +{ + inode_maybe_inc_iversion(inode, true); +} + +/** + * inode_iversion_need_inc - is the i_version in need of being incremented? + * @inode: inode to check + * + * Returns whether the inode->i_version counter needs incrementing on the next + * change. Just fetch the value and check the QUERIED flag. + */ +static inline bool +inode_iversion_need_inc(struct inode *inode) +{ + return inode_peek_iversion_raw(inode) & I_VERSION_QUERIED; +} + +/** + * inode_inc_iversion_raw - forcibly increment raw i_version + * @inode: inode that needs to be updated + * + * Forcbily increment the raw i_version field. This always results in a change + * to the raw value. + * + * NFS will use the i_version field to store the value from the server. It + * mostly treats it as opaque, but in the case where it holds a write + * delegation, it must increment the value itself. This function does that. + */ +static inline void +inode_inc_iversion_raw(struct inode *inode) +{ + atomic64_inc(&inode->i_version); +} + +/** + * inode_peek_iversion - read i_version without flagging it to be incremented + * @inode: inode from which i_version should be read + * + * Read the inode i_version counter for an inode without registering it as a + * query. + * + * This is typically used by local filesystems that need to store an i_version + * on disk. In that situation, it's not necessary to flag it as having been + * viewed, as the result won't be used to gauge changes from that point. + */ +static inline u64 +inode_peek_iversion(const struct inode *inode) +{ + return inode_peek_iversion_raw(inode) >> I_VERSION_QUERIED_SHIFT; +} + +/** + * inode_query_iversion - read i_version for later use + * @inode: inode from which i_version should be read + * + * Read the inode i_version counter. This should be used by callers that wish + * to store the returned i_version for later comparison. This will guarantee + * that a later query of the i_version will result in a different value if + * anything has changed. + * + * In this implementation, we fetch the current value, set the QUERIED flag and + * then try to swap it into place with a cmpxchg, if it wasn't already set. If + * that fails, we try again with the newly fetched value from the cmpxchg. + */ +static inline u64 +inode_query_iversion(struct inode *inode) +{ + u64 cur, old, new; + + cur = inode_peek_iversion_raw(inode); + for (;;) { + /* If flag is already set, then no need to swap */ + if (cur & I_VERSION_QUERIED) { + /* + * This barrier (and the implicit barrier in the + * cmpxchg below) pairs with the barrier in + * inode_maybe_inc_iversion(). + */ + smp_mb(); + break; + } + + new = cur | I_VERSION_QUERIED; + old = atomic64_cmpxchg(&inode->i_version, cur, new); + if (likely(old == cur)) + break; + cur = old; + } + return cur >> I_VERSION_QUERIED_SHIFT; +} + +/** + * inode_cmp_iversion_raw - check whether the raw i_version counter has changed + * @inode: inode to check + * @old: old value to check against its i_version + * + * Compare the current raw i_version counter with a previous one. Returns false + * if they are the same or true if they are different. + */ +static inline bool +inode_cmp_iversion_raw(const struct inode *inode, u64 old) +{ + return inode_peek_iversion_raw(inode) != old; +} + +/** + * inode_cmp_iversion - check whether the i_version counter has changed + * @inode: inode to check + * @old: old value to check against its i_version + * + * Compare an i_version counter with a previous one. Returns false if they are + * the same, and true if they are different. + * + * Note that we don't need to set the QUERIED flag in this case, as the value + * in the inode is not being recorded for later use. + */ +static inline bool +inode_cmp_iversion(const struct inode *inode, u64 old) +{ + return inode_peek_iversion(inode) != old; +} +#endif |