diff options
Diffstat (limited to 'include/linux/iversion.h')
| -rw-r--r-- | include/linux/iversion.h | 208 |
1 files changed, 153 insertions, 55 deletions
diff --git a/include/linux/iversion.h b/include/linux/iversion.h index f268828f9f7e..858463fca249 100644 --- a/include/linux/iversion.h +++ b/include/linux/iversion.h @@ -5,6 +5,8 @@ #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 @@ -33,86 +35,171 @@ * 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 - * @new: new i_version value to set + * @val: new i_version value to set * - * Set @inode's i_version field to @new. This function is for use by + * 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 new) +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) { - inode->i_version = new; + return atomic64_read(&inode->i_version); } /** * inode_set_iversion - set i_version to a particular value * @inode: inode to set - * @new: new i_version value to set + * @val: new i_version value to set * - * Set @inode's i_version field to @new. This function is for filesystems with - * a kernel-managed i_version. + * 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. * - * For now, this just does the same thing as the _raw variant. + * 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 new) +inode_set_iversion(struct inode *inode, u64 val) { - inode_set_iversion_raw(inode, new); + inode_set_iversion_raw(inode, val << I_VERSION_QUERIED_SHIFT); } /** - * inode_set_iversion_queried - set i_version to a particular value and set - * flag to indicate that it has been viewed + * inode_set_iversion_queried - set i_version to a particular value as quereied * @inode: inode to set - * @new: new i_version value to set + * @val: new i_version value to set * - * When loading in an i_version value from a backing store, we typically don't - * know whether it was previously viewed before being stored or not. Thus, we - * must assume that it was, to ensure that any changes will result in the - * value changing. + * Set @inode's i_version field to @val, and flag it for increment on the next + * change. * - * This function will set the inode's i_version, and possibly flag the value - * as if it has already been viewed at least once. + * Filesystems that persistently store the i_version on disk should use this + * when loading an existing inode from disk. * - * For now, this just does what inode_set_iversion does. + * 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 new) +inode_set_iversion_queried(struct inode *inode, u64 val) { - inode_set_iversion(inode, new); + 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 + * @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. * - * In this implementation, we always increment it after taking the i_lock to - * ensure that we don't race with other incrementors. + * If "force" is set or the QUERIED flag is set, then ensure that we increment + * the value, and clear the queried flag. * - * Returns true if counter was bumped, and false if it wasn't. + * 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) { - atomic64_t *ivp = (atomic64_t *)&inode->i_version; + 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; - atomic64_inc(ivp); + /* 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; } @@ -135,31 +222,12 @@ inode_inc_iversion(struct inode *inode) * @inode: inode to check * * Returns whether the inode->i_version counter needs incrementing on the next - * change. - * - * For now, we assume that it always does. + * change. Just fetch the value and check the QUERIED flag. */ static inline bool inode_iversion_need_inc(struct inode *inode) { - return true; -} - -/** - * 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 inode->i_version; + return inode_peek_iversion_raw(inode) & I_VERSION_QUERIED; } /** @@ -176,7 +244,7 @@ inode_peek_iversion_raw(const struct inode *inode) static inline void inode_inc_iversion_raw(struct inode *inode) { - inode_inc_iversion(inode); + atomic64_inc(&inode->i_version); } /** @@ -193,7 +261,7 @@ inode_inc_iversion_raw(struct inode *inode) static inline u64 inode_peek_iversion(const struct inode *inode) { - return inode_peek_iversion_raw(inode); + return inode_peek_iversion_raw(inode) >> I_VERSION_QUERIED_SHIFT; } /** @@ -205,12 +273,35 @@ inode_peek_iversion(const struct inode *inode) * that a later query of the i_version will result in a different value if * anything has changed. * - * This implementation just does a peek. + * 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) { - return inode_peek_iversion(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; } /** @@ -233,11 +324,18 @@ inode_cmp_iversion_raw(const struct inode *inode, u64 old) * @old: old value to check against its i_version * * Compare an i_version counter with a previous one. Returns 0 if they are - * the same or non-zero if they are different. + * the same, a positive value if the one in the inode appears newer than @old, + * and a negative value if @old appears to be newer than the one in the + * inode. + * + * 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 s64 inode_cmp_iversion(const struct inode *inode, u64 old) { - return (s64)inode_peek_iversion(inode) - (s64)old; + return (s64)(inode_peek_iversion_raw(inode) & ~I_VERSION_QUERIED) - + (s64)(old << I_VERSION_QUERIED_SHIFT); } #endif |