include/linux/iversion.h

/* 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