I'm proposing to expose the inode change attribute via statx [1]. Document what this value means and what an observer can infer from a change in its value. Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> [1]: https://lore.kernel.org/linux-nfs/20220826214703.134870-1-jlayton@xxxxxxxxxx/T/#t --- man2/statx.2 | 8 ++++++++ man7/inode.7 | 34 ++++++++++++++++++++++++++++++++++ 2 files changed, 42 insertions(+) v3: Move most verbiage to inode(7) Clarify that this must be monotonically increasing Flesh out usage discussion Mention issues with value moving backward and how to combat them diff --git a/man2/statx.2 b/man2/statx.2 index 0d1b4591f74c..d98d5148a442 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 @@ -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 diff --git a/man7/inode.7 b/man7/inode.7 index 9b255a890720..80c2ed4acccd 100644 --- a/man7/inode.7 +++ b/man7/inode.7 @@ -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 -- 2.37.3
