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@xxxxxxxxxx>
