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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> > >
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