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Message-ID: <7901f60f7ea67d757caa5ba26588f01f308caaac.camel@kernel.org>
Subject: Re: [PATCH] vfs: report an inode version in statx for IS_I_VERSION
 inodes
From:   Jeff Layton <jlayton@kernel.org>
To:     Dave Chinner <david@fromorbit.com>
Cc:     viro@zeniv.linux.org.uk, linux-api@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
        David Howells <dhowells@redhat.com>,
        Frank Filz <ffilzlnx@mindspring.com>
Date:   Wed, 24 Aug 2022 06:17:22 -0400
In-Reply-To: <20220823215333.GC3144495@dread.disaster.area>
References: <20220819115641.14744-1-jlayton@kernel.org>
         <20220823215333.GC3144495@dread.disaster.area>
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On Wed, 2022-08-24 at 07:53 +1000, Dave Chinner wrote:
> On Fri, Aug 19, 2022 at 07:56:41AM -0400, Jeff Layton wrote:
> > From: Jeff Layton <jlayton@redhat.com>
> >=20
> > The NFS server and IMA both rely heavily on the i_version counter, but
> > it's largely invisible to userland, which makes it difficult to test it=
s
> > behavior. This value would also be of use to userland NFS servers, and
> > other applications that want a reliable way to know if there was an
> > explicit change to an inode since they last checked.
> >=20
> > Claim one of the spare fields in struct statx to hold a 64-bit inode
> > version attribute. This value must change with any explicit, observeabl=
e
> > metadata or data change. Note that atime updates are excluded from this=
,
> > unless it is due to an explicit change via utimes or similar mechanism.
> >=20
> > When statx requests this attribute on an IS_I_VERSION inode, do an
> > inode_query_iversion and fill the result in the field. Also, update the
> > test-statx.c program to display the inode version and the mountid.
> >=20
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Frank Filz <ffilzlnx@mindspring.com>
> > Signed-off-by: Jeff Layton <jlayton@kernel.org>
>=20
> NAK.
>=20
> THere's no definition of what consitutes an "inode change" and this
> exposes internal filesystem implementation details (i.e. on disk
> format behaviour) directly to userspace. That means when the
> internal filesystem behaviour changes, userspace applications will
> see changes in stat->ino_version changes and potentially break them.
>=20
> We *need a documented specification* for the behaviour we are exposing to
> userspace here, and then individual filesystems needs to opt into
> providing this information as they are modified to conform to the
> behaviour we are exposing directly to userspsace.
>=20
> Jeff - can you please stop posting iversion patches to different
> subsystems as individual, unrelated patchsets and start posting all
> the changes - statx, ext4, xfs, man pages, etc as a single patchset
> so the discussion can be centralised in one place and not spread
> over half a dozen disconnected threads?
>=20


Sure. Give me a few days and I'll post a more coherent set of patches.

Thanks,
--=20
Jeff Layton <jlayton@kernel.org>