Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 Jul 2007 18:09:40 -0400 Mingming Cao <cmm@us.ibm.com> wrote:

> On Tue, 2007-07-10 at 16:30 -0700, Andrew Morton wrote:
> > On Sun, 01 Jul 2007 03:37:04 -0400
> > Mingming Cao <cmm@us.ibm.com> wrote:
> > 
> > > This patch converts the 32-bit i_version in the generic inode to a 64-bit
> > > i_version field.
> > > 
> > 
> > That's obvious from the patch.  But what was the reason for making this
> > (unrelated to ext4) change?
> > 
> 
> The need is came from NFSv4
> 
> On Fri, 2007-05-25 at 18:25 +0200, Jean noel Cordenner wrote: 
> > Hi,
> > 
> > This is an update of the i_version patch.
> > The i_version field is a 64bit counter that is set on every inode
> > creation and that is incremented every time the inode data is modified
> > (similarly to the "ctime" time-stamp).
> > The aim is to fulfill a NFSv4 requirement for rfc3530:
> > "5.5.  Mandatory Attributes - Definitions
> > Name		#	DataType   Access   Description
> > ___________________________________________________________________
> > change		3	uint64       READ     A value created by the
> > 		server that the client can use to determine if file
> > 		data, directory contents or attributes of the object
> > 		have been modified.  The servermay return the object's
> > 		time_metadata attribute for this attribute's value but
> > 		only if the filesystem object can not be updated more
> > 		frequently than the resolution of time_metadata.
> > "
> > 
> 
> > Please update the changelog for this.
> > 
> 
> Is above description clear to you?
> 

Yes, thanks.  It doesn't actually tell us why we want to implement
this attribute and it doesn't tell us what the implications of failing
to do so are, but I guess we can take that on trust from the NFS guys.

But I suspect the ext4 implementation doesn't actually do this.  afaict we
won't update i_version for file overwrites (especially if s_time_gran can
indeed be 1,000,000,000) and of course for MAP_SHARED modifications.  What
would be the implications of this?

And how does the NFS server know that the filesystem implements i_version? 
Will a zero-value of i_version have special significance, telling the
server to not send this attribute, perhaps?