statx, inode: document the STATX_INO_VERSION field

index 0d1b4591f74c..d98d5148a442 100644
@@ -62,6 +62,7 @@ struct statx {
     __u32 stx_dev_major;   /* Major ID */
     __u32 stx_dev_minor;   /* Minor ID */
     __u64 stx_mnt_id;      /* Mount ID */
+    __u64 stx_ino_version; /* Inode change attribute */
 };
 .EE
 .in

@@ -247,6 +248,7 @@ STATX_BTIME	Want stx_btime
 STATX_ALL	The same as STATX_BASIC_STATS | STATX_BTIME.
 	It is deprecated and should not be used.
 STATX_MNT_ID	Want stx_mnt_id (since Linux 5.8)
+STATX_INO_VERSION	Want stx_ino_version (DRAFT)
 .TE
 .in
 .PP

@@ -407,10 +409,16 @@ This is the same number reported by
 .BR name_to_handle_at (2)
 and corresponds to the number in the first field in one of the records in
 .IR /proc/self/mountinfo .
+.TP
+.I stx_ino_version
+The inode version, also known as the inode change attribute. See
+.BR inode (7)
+for details.
 .PP
 For further information on the above fields, see
 .BR inode (7).
 .\"
+.TP
 .SS File attributes
 The
 .I stx_attributes

index 9b255a890720..80c2ed4acccd 100644
@@ -184,6 +184,12 @@ Last status change timestamp (ctime)
 This is the file's last status change timestamp.
 It is changed by writing or by setting inode information
 (i.e., owner, group, link count, mode, etc.).
+.TP
+Inode version (i_version)
+(not returned in the \fIstat\fP structure); \fIstatx.stx_ino_version\fP
+.IP
+This is the inode change counter. See the discussion of
+\fBthe inode version counter\fP, below.
 .PP
 The timestamp fields report time measured with a zero point at the
 .IR Epoch ,

@@ -424,6 +430,34 @@ on a directory means that a file
 in that directory can be renamed or deleted only by the owner
 of the file, by the owner of the directory, and by a privileged
 process.
+.SS The inode version counter
+.PP
+The
+.I statx.stx_ino_version
+field is the inode change counter. Any operation that would result in a
+change to \fIstatx.stx_ctime\fP must result in an increase to this value.
+The value must increase even in the case where the ctime change is not
+evident due to coarse timestamp granularity.
+.PP
+An observer cannot infer anything from amount of increase about the
+nature or magnitude of the change. If the returned value is different
+from the last time it was checked, then something has made an explicit
+data and/or metadata change to the inode.
+.PP
+In the event of a system crash, this value can appear to go backward,
+if it were queried before ever being written to the backing store. If
+the value were then incremented again after restart, then an observer
+could miss noticing a change.
+.PP
+In order to guard against this, it is recommended to also watch the
+\fIstatx.stx_ctime\fP for changes when watching this value. As long as the
+system clock doesn't jump backward during the crash, an observer can be
+reasonably sure that the i_version and ctime together represent a unique inode
+state.
+.PP
+The i_version is a Linux extension and is not supported by all filesystems.
+The application must verify that the \fISTATX_INO_VERSION\fP bit is set in the
+returned \fIstatx.stx_mask\fP before relying on this field.
 .SH STANDARDS
 If you need to obtain the definition of the
 .I blkcnt_t