/* 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_eq_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_eq_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 true
* if they are the same or false if they are different.
*/
static inline bool
inode_eq_iversion_raw(const struct inode *inode, u64 old)
{
return inode_peek_iversion_raw(inode) == old;
}
/**
* inode_eq_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 true if they are
* the same, and false 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_eq_iversion(const struct inode *inode, u64 old)
{
return inode_peek_iversion(inode) == old;
}
#endif
