linux-kernel - Re: [PATCH v3 1/7] iversion: update comments with info about atime updates

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Message-ID: <20220830144430.GD26330@fieldses.org>
Date:   Tue, 30 Aug 2022 10:44:30 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Jeff Layton <jlayton@...nel.org>
Cc:     NeilBrown <neilb@...e.de>, Dave Chinner <david@...morbit.com>,
        tytso@....edu, adilger.kernel@...ger.ca, djwong@...nel.org,
        trondmy@...merspace.com, viro@...iv.linux.org.uk,
        zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com,
        lczerner@...hat.com, jack@...e.cz, brauner@...nel.org,
        linux-api@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ceph@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org,
        Colin Walters <walters@...bum.org>
Subject: Re: [PATCH v3 1/7] iversion: update comments with info about atime
 updates

On Tue, Aug 30, 2022 at 09:50:02AM -0400, Jeff Layton wrote:
> On Tue, 2022-08-30 at 09:24 -0400, J. Bruce Fields wrote:
> > On Tue, Aug 30, 2022 at 07:40:02AM -0400, Jeff Layton wrote:
> > > Yes, saying only that it must be different is intentional. What we
> > > really want is for consumers to treat this as an opaque value for the
> > > most part [1]. Therefore an implementation based on hashing would
> > > conform to the spec, I'd think, as long as all of the relevant info is
> > > part of the hash.
> > 
> > It'd conform, but it might not be as useful as an increasing value.
> > 
> > E.g. a client can use that to work out which of a series of reordered
> > write replies is the most recent, and I seem to recall that can prevent
> > unnecessary invalidations in some cases.
> > 
> 
> That's a good point; the linux client does this. That said, NFSv4 has a
> way for the server to advertise its change attribute behavior [1]
> (though nfsd hasn't implemented this yet).

It was implemented and reverted.  The issue was that I thought nfsd
should mix in the ctime to prevent the change attribute going backwards
on reboot (see fs/nfsd/nfsfh.h:nfsd4_change_attribute()), but Trond was
concerned about the possibility of time going backwards.  See
1631087ba872 "Revert "nfsd4: support change_attr_type attribute"".
There's some mailing list discussion to that I'm not turning up right
now.

Did NFSv4 add change_attr_type because some implementations needed the
unordered case, or because they realized ordering was useful but wanted
to keep backwards compatibility?  I don't know which it was.

--b.

> We don't have a good way to
> do that in userland for now.
> 
> This is another place where fsinfo() would have been nice to have. I
> think until we have something like that, we'd want to keep our promises
> to userland to a minimum.
> 
> [1]: https://www.rfc-editor.org/rfc/rfc7862.html#section-12.2.3 . I
> guess I should look at plumbing this in for IS_I_VERSION inodes...
> 
> -- 
> Jeff Layton <jlayton@...nel.org>
> 
>