linux-ext4 - Re: [RFC PATCH v2] statx, inode: document the new STATX_INO

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Date:   Thu, 01 Sep 2022 18:12:33 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, djwong@...nel.org,
        david@...morbit.com, trondmy@...merspace.com, neilb@...e.de,
        viro@...iv.linux.org.uk, zohar@...ux.ibm.com, xiubli@...hat.com,
        chuck.lever@...cle.com, lczerner@...hat.com, jack@...e.cz,
        bfields@...ldses.org, brauner@...nel.org,
        linux-man@...r.kernel.org, linux-api@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH v2] statx, inode: document the new STATX_INO_VERSION
 field

* Jeff Layton:

> @@ -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.

What happens if the file system does not support i_version?

> 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.

What is the wraparound behavior for i_version?  Does it use the full
64-bit range?

If the system crashes without flushing disks, is it possible to observe
new file contents without a change of i_version?

Thanks,
Florian