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. NB: this will probably have conflicts with the STATX_DIOALIGN doc patches, but we should be able to resolve those before merging anything. Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> [1]: https://lore.kernel.org/linux-nfs/20220826214703.134870-1-jlayton@xxxxxxxxxx/T/#t --- man2/statx.2 | 13 +++++++++++++ man7/inode.7 | 36 ++++++++++++++++++++++++++++++++++++ 2 files changed, 49 insertions(+) v6: incorporate Neil's suggestions clarify how well-behaved filesystems should order things diff --git a/man2/statx.2 b/man2/statx.2 index 0d1b4591f74c..ee7005334a2f 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_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_VERSION Want stx_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_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 @@ -489,6 +497,11 @@ without an explicit See .BR mmap (2) for more information. +.TP +.BR STATX_ATTR_VERSION_MONOTONIC " (since Linux 6.?)" +The stx_version value monotonically increases over time and will never appear +to go backward, even in the event of a crash. This can allow an application to +make a better determination about ordering when viewing different versions. .SH RETURN VALUE On success, zero is returned. On error, \-1 is returned, and diff --git a/man7/inode.7 b/man7/inode.7 index 9b255a890720..e8adb63b1f6a 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 (version) +(not returned in the \fIstat\fP structure); \fIstatx.stx_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 +430,36 @@ 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 \fIstatx.stx_version\fP field is the inode change counter. Any operation +that could result in a change to \fIstatx.stx_ctime\fP must result in an +increase to this value. Soon after a change has been made, an stx_version value +should appear to be larger than previous readings. This is the case even +when a 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. In fact, a single increment can reflect +multiple discrete changes if the value was not checked while those changes +were being processed. +.PP +Changes to stx_version are not necessarily atomic with the change itself, but +well-behaved filesystems should increment stx_version after a change has been +made visible to observers rather than before. This is especially important for +read-caching algorithms which could be fooled into associating a newer +stx_version with an older version of data. Note that this does leave a window +of time where a change may be visible, but the old stx_version is still being +reported. +.PP +In the event of a system crash, this value can appear to go backward if it was +queried before ever being written to the backing store. Applications that +persist stx_version values across a reboot should take care to mitigate this. +If the filesystem reports \fISTATX_ATTR_VERSION_MONOTONIC\fP in +\fIstatx.stx_attributes\fP, then it is not subject to this problem. +.PP +The stx_version is a Linux extension and is not supported by all filesystems. +The application must verify that the \fISTATX_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
