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From:   David Howells <dhowells@redhat.com>
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To:     Jeff Layton <jlayton@kernel.org>
Cc:     dhowells@redhat.com, Christian Brauner <brauner@kernel.org>,
        viro@zeniv.linux.org.uk, linux-afs@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
        ceph-devel@vger.kernel.org
Subject: Re: [PATCH 1/4] vfs: report change attribute in statx for IS_I_VERSION inodes
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Date:   Tue, 16 Aug 2022 16:15:03 +0100
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Jeff Layton <jlayton@kernel.org> wrote:

> I think we'll just have to ensure that before we expose this for any
> filesystem that it conforms to some minimum standards. i.e.: it must
> change if there are data or metadata changes to the inode, modulo atime
> changes due to reads on regular files or readdir on dirs.
> 
> The local filesystems, ceph and NFS should all be fine. I guess that
> just leaves AFS. If it can't guarantee that, then we might want to avoid
> exposing the counter for it.

AFS monotonically increments the counter on data changes; doesn't make any
change for metadata changes (other than the file size).

But you can't assume NFS works as per your suggestion as you don't know what's
backing it (it could be AFS, for example - there's a converter for that).

Further, for ordinary disk filesystems, two data changes may get elided and
only increment the counter once.  And then there's mmap...

It might be better to reduce the scope of your definition and just say that it
must change if there's a data change and may also be changed if there's a
metadata change.

David