[RFC PATCH v2] statx, inode: document the new STATX_INO_VERSION field

[Jeff Layton <jlayton@kernel.org>]

I'm proposing to expose the inode change attribute via statx [1]. Document
what this value means and what an observer can infer from it changing.

Signed-off-by: Jeff Layton <jlayton@kernel.org>

[1]: https://lore.kernel.org/linux-nfs/20220826214703.134870-1-jlayton@kernel.org/T/#t
---
 man2/statx.2 | 17 +++++++++++++++++
 man7/inode.7 | 12 ++++++++++++
 2 files changed, 29 insertions(+)

v2: revised the definition to be more strict, since that seemed to be
    consensus on desired behavior. Spurious i_version bumps would now
    be considered bugs, by this definition.

diff --git a/man2/statx.2 b/man2/statx.2
index 0d1b4591f74c..493e4e234809 100644
--- a/man2/statx.2
+++ b/man2/statx.2
@@ -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
@@ -411,6 +413,21 @@ and corresponds to the number in the first field in one of the records in
 For further information on the above fields, see
 .BR inode (7).
 .\"
+.TP
+.I stx_ino_version
+The inode version, also known as the inode change attribute. This
+value must change any time there is an inode status change. Any
+operation that would cause the
+.I stx_ctime
+to change must also cause
+.I stx_ino_version
+to change, even when there is no apparent change to the
+.I stx_ctime
+due to coarse timestamp granularity.
+.IP
+An observer cannot infer anything about the nature or magnitude of the change
+from the value of this field. A change in this value only indicates that
+there has been an explicit change in the inode.
 .SS File attributes
 The
 .I stx_attributes
diff --git a/man7/inode.7 b/man7/inode.7
index 9b255a890720..d5e0890a52c0 100644
--- a/man7/inode.7
+++ b/man7/inode.7
@@ -184,6 +184,18 @@ 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 attribute. Any operation that would result in a change
+to \fIstatx.stx_ctime\fP must result in a change to this value. The value must
+change even in the case where the ctime change is not evident due to coarse
+timestamp granularity.
+.IP
+An observer cannot infer anything from the returned value 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 change to the inode.
 .PP
 The timestamp fields report time measured with a zero point at the
 .IR Epoch ,
-- 
2.37.2