linux-ext4 - Re: [PATCH v3 1/7] iversion: update comments with info about atime updates

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Date:   Tue, 30 Aug 2022 09:50:02 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     "J. Bruce Fields" <bfields@...ldses.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, 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). 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>