TOMOYO Linux Cross Reference
Linux/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 larger to observers if there was an explicit change to the inode's
   * data or metadata since it was last queried.
   *
   * An explicit change is one that would ordinarily result in a change to the
   * inode status change time (aka ctime). i_version must appear to change, even
   * if the ctime does not (since the whole point is to avoid missing updates due
   * to timestamp granularity). If POSIX or other relevant spec mandates that the
   * ctime must change due to an operation, then the i_version counter must be
   * incremented as well.
   *
   * Making the i_version update completely atomic with the operation itself would
   * be prohibitively expensive. Traditionally the kernel has updated the times on
   * directories after an operation that changes its contents. For regular files,
   * the ctime is usually updated before the data is copied into the cache for a
   * write. This means that there is a window of time when an observer can
   * associate a new timestamp with old file contents. Since the purpose of the
   * i_version is to allow for better cache coherency, the i_version must always
   * be updated after the results of the operation are visible. Updating it before
   * and after a change is also permitted. (Note that no filesystems currently do
   * this. Fixing that is a work-in-progress).
   *
   * 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_max_iversion_raw - update i_version new value is larger
  * @inode: inode to set
  * @val: new i_version to set
  *
  * Some self-managed filesystems (e.g Ceph) will only update the i_version
  * value if the new value is larger than the one we already have.
  */
 static inline void
 inode_set_max_iversion_raw(struct inode *inode, u64 val)
 {
         u64 cur = inode_peek_iversion_raw(inode);
 
         do {
                 if (cur > val)
                         break;
         } while (!atomic64_try_cmpxchg(&inode->i_version, &cur, val));
 }
 
 /**
  * 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);
 }
 
 bool inode_maybe_inc_iversion(struct inode *inode, bool force);
 
 /**
  * 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;
 }
 
 /*
  * For filesystems without any sort of change attribute, the best we can
  * do is fake one up from the ctime:
  */
 static inline u64 time_to_chattr(const struct timespec64 *t)
 {
         u64 chattr = t->tv_sec;
 
         chattr <<= 32;
         chattr += t->tv_nsec;
         return chattr;
 }
 
 u64 inode_query_iversion(struct inode *inode);
 
 /**
  * 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
 

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