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Message-ID: <83d07cc4f7fe2ca9976d3f418e5137f354e933a4.camel@kernel.org>
Subject: Re: [PATCH 1/4] vfs: report change attribute in statx for
 IS_I_VERSION inodes
From:   Jeff Layton <jlayton@kernel.org>
To:     David Howells <dhowells@redhat.com>
Cc:     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
Date:   Tue, 16 Aug 2022 11:32:24 -0400
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On Tue, 2022-08-16 at 16:15 +0100, David Howells wrote:
> Jeff Layton <jlayton@kernel.org> wrote:
>=20
> > 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.
> >=20
> > 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 avoi=
d
> > exposing the counter for it.
>=20
> AFS monotonically increments the counter on data changes; doesn't make an=
y
> change for metadata changes (other than the file size).
>=20
> But you can't assume NFS works as per your suggestion as you don't know w=
hat's
> backing it (it could be AFS, for example - there's a converter for that).
>=20

In that case, the NFS server must synthesize a proper change attr. The
NFS spec mandates that it change on most metadata changes.

> Further, for ordinary disk filesystems, two data changes may get elided a=
nd
> only increment the counter once.
>=20

Not a problem as long as nothing queried the counter in between the
changes.

> And then there's mmap...
>=20

Not sure how that matters here.

> It might be better to reduce the scope of your definition and just say th=
at it
> must change if there's a data change and may also be changed if there's a
> metadata change.
>=20

I'd prefer that we mandate that it change on metadata changes as well.
That's what most of the in-kernel users want, and what most of the
existing filesystems provide. If AFS can't give that guarantee then we
can just omit exposing i_version on it.
--=20
Jeff Layton <jlayton@kernel.org>