Re: [PATCH 2/2] nfsd: implement chage_attr_type attribute

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 13, 2014 at 07:43:33PM -0500, Trond Myklebust wrote:
> On Thu, Nov 13, 2014 at 6:54 PM, Dave Chinner <david@fromorbit.com> wrote:
> > On Thu, Nov 13, 2014 at 08:02:43AM -0500, Trond Myklebust wrote:
> >> On Wed, Nov 12, 2014 at 7:28 PM, Dave Chinner <david@fromorbit.com> wrote:
> >> > On Wed, Nov 12, 2014 at 09:26:16AM -0500, Trond Myklebust wrote:
> >> >> On Wed, Nov 12, 2014 at 5:24 AM, Christoph Hellwig <hch@lst.de> wrote:
> >> >> > On Wed, Nov 12, 2014 at 09:27:10AM +1100, Dave Chinner wrote:
> >> >> >> To clarify what Christoph wrote, XFS updates i_version is updated
> >> >> >> once per transaction that modifies the inode. So if a VFS level
> >> >> >> operation results in multiple transactions then each transaction
> >> >> >> will but the version.
> >> >> >>
> >> >> >> It was implemented that way because nobody could tell me what the
> >> >> >> actual granularity requirement for change detection was.  Hence what
> >> >> >> I implemented was "be able to detect any persistent change that is
> >> >> >> made" to cover all bases.
> >
> > FWIW, ext4 takes the same approach.  See Ted's post today:
> >
> > http://www.spinics.net/lists/linux-ext4/msg46194.html
> >
> > "The inode_inc_iversion() in mark4_ext4_iloc_dirty() is probably not
> > necessary, since we already should be incrementing i_version whenever
> > ctime and mtime gets updated.  The inode_inc_iversion() there is more
> > of a "belt and suspenders" safety thing, on the theory that the extra
> > bump in i_version won't hurt anything."
> >
> 
> It will hurt if it causes all the NFS clients to blow their caches
> unnecessarily.

Not my problem. We've just implemented what we were asked to
implement.

> Who asked for this?

The only discussion where actual specifications were enumerated was
during a thread about using i_version in the integrity measurement
code (IMA subsystem). The NFSv4 requirements for the change counter
were expressed here:

https://lkml.org/lkml/2012/1/5/408

Don't blame us for implementing the vague "changes every time"
requirements in a way that results in no chance of a persistent
change to either data or metadata being missed by the filesystem.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com