Re: [PATCH v5 01/19] fs: new API for handling inode->i_version

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2018-01-18 at 16:38 -0500, J. Bruce Fields wrote:
> On Tue, Jan 09, 2018 at 09:10:41AM -0500, Jeff Layton wrote:
> > --- /dev/null
> > +++ b/include/linux/iversion.h
> > @@ -0,0 +1,236 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _LINUX_IVERSION_H
> > +#define _LINUX_IVERSION_H
> > +
> > +#include <linux/fs.h>
> > +
> > +/*
> > + * The change attribute (i_version) is mandated by NFSv4 and is mostly for
> > + * knfsd, but is also used for other purposes (e.g. IMA). The i_version must
> > + * appear different to observers if there was a change to the inode's data or
> > + * metadata since it was last queried.
> > + *
> > + * Observers see the i_version as a 64-bit number that never changes.
> 
> I don't understand that sentence.
> 

That's because it's utter nonsense. I noticed that the other day and
fixed it in my tree. It now reads:

* Observers see the i_version as a 64-bit number that never decreases.

> > If it
> > + * remains the same since it was last checked, then nothing has changed in the
> > + * inode. If it's different then something has changed. Observers cannot infer
> > + * anything about the nature or magnitude of the changes from the value, only
> > + * that the inode has changed in some fashion.
> 
> As we've discussed before, there may be brief windows where the first
> two statements aren't quite correct.  I think that would be worth a
> mention if we can keep it concise.  Maybe add something like this?:
> 
> 	It may be impractical for filesystems to keep i_version updates
> 	atomic with respect to the changes that cause them.  They
> 	should, however, guarantee that i_version updates are never
> 	visible before the changes that caused them.  Also, i_version
> 	updates should never be delayed longer than it takes the
> 	original change to reach disk.

That makes sense. I added it in pretty much verbatim. I think we mostly
follow the latter should already.

> Or maybe those details are best left to documentation on the relevant
> parts of the api below (maybe inode_maybe_inc_iversion?).
> 
> I dunno if it's also worth mentioning that nfsd doesn't actually use the
> raw i_version--it mixes it with ctime to prevent i_version reuse after
> reboot.  Presumably that doesn't matter to IMA since it doesn't compare
> i_version across reboots.
> 

I think I won't document that here. nfsd is a consumer of i_version.
What it does with it is sort of its own business. Might be good to have
a comment blurb in the nfsd code about it though.

> The documentation here is all very helpful, thanks.

Thanks for all of the suggestions so far!
-- 
Jeff Layton <jlayton@kernel.org>