Re: [PATCH] iversion: update comments with info about atime updates

[Jeff Layton]

On Mon, 2022-08-22 at 13:39 -0400, Mimi Zohar wrote:
> On Mon, 2022-08-22 at 12:22 -0400, Jeff Layton wrote:
> > On Mon, 2022-08-22 at 11:40 -0400, Mimi Zohar wrote:
> > > On Mon, 2022-08-22 at 09:33 -0400, Jeff Layton wrote:
> > > > Add an explicit paragraph codifying that atime updates due to reads
> > > > should not be counted against the i_version counter. None of the
> > > > existing subsystems that use the i_version want those counted, and
> > > > there is an easy workaround for those that do.
> > > > 
> > > > Cc: NeilBrown <neilb@xxxxxxx>
> > > > Cc: Trond Myklebust <trondmy@xxxxxxxxxxxxxxx>
> > > > Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> > > > Link: https://lore.kernel.org/linux-xfs/166086932784.5425.17134712694961326033@xxxxxxxxxxxxxxxxxxxxx/#t
> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > ---
> > > >  include/linux/iversion.h | 10 ++++++++--
> > > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> > > > index 3bfebde5a1a6..da6cc1cc520a 100644
> > > > --- a/include/linux/iversion.h
> > > > +++ b/include/linux/iversion.h
> > > > @@ -9,8 +9,8 @@
> > > >   * ---------------------------
> > > >   * 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.
> > > > + * appear different to observers if there was an explicit 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 decreases. If it
> > > >   * remains the same since it was last checked, then nothing has changed in the
> > > > @@ -18,6 +18,12 @@
> > > >   * anything about the nature or magnitude of the changes from the value, only
> > > >   * that the inode has changed in some fashion.
> > > >   *
> > > > + * Note that atime updates due to reads or similar activity do _not_ represent
> > > > + * an explicit change to the inode. If the only change is to the atime and it
> > > 
> > > Thanks, Jeff.  The ext4 patch increments i_version on file metadata
> > > changes.  Could the wording here be more explicit to reflect changes
> > > based on either inode data or metadata changes?b
> > > 
> > > 
> > 
> > Thanks Mimi,
> > 
> > Care to suggest some wording?
> > 
> > The main issue we have is that ext4 and xfs both increment i_version on
> > atime updates due to reads. I have patches in flight to fix those, but
> > going forward, we want to ensure that i_version gets incremented on all
> > changes _except_ for atime updates.
> > 
> > The best wording we have at the moment is what Trond suggested, which is
> > to classify the changes to the inode as "explicit" (someone or something
> > made a deliberate change to the inode) and "implicit" (the change to the
> > inode was due to activity such as reads that don't actually change
> > anything).
> > 
> > Is there a better way to describe this?
> 
> "explicit change to the inode" probably implies both the inode file
> data and metadata, but let's call it out by saying "an explicit change
> to either the inode data or metadata".
> 
> > 
> > > > + * wasn't set via utimes() or a similar mechanism, then i_version should not be
> > > > + * incremented. If an observer cares about atime updates, it should plan to
> > > > + * fetch and store them in conjunction with the i_version.
> > > > + *
> > > >   * Not all filesystems properly implement the i_version counter. Subsystems that
> > > >   * want to use i_version field on an inode should first check whether the
> > > >   * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION macro).
> > > 
> > > 
> > 
> 
> 

Thanks Mimi,

Here's what I have now. I'll plan to send a v2 patch once others have
had a chance to comment as well.

-- Jeff

diff --git a/include/linux/iversion.h b/include/linux/iversion.h
index 3bfebde5a1a6..524abd372100 100644
--- a/include/linux/iversion.h
+++ b/include/linux/iversion.h
@@ -9,8 +9,8 @@
  * ---------------------------
  * 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.
+ * appear different to observers if there was an explicit 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 decreases. If it
  * remains the same since it was last checked, then nothing has changed in the
@@ -18,6 +18,13 @@
  * anything about the nature or magnitude of the changes from the value, only
  * that the inode has changed in some fashion.
  *
+ * Note that atime updates due to reads or similar activity do not represent
+ * an explicit change to the inode data or metadata. If the only change is to
+ * the atime and it wasn't set via utimes() or a similar mechanism, then
+ * i_version should not be incremented. If an observer cares about atime
+ * updates, it should plan to fetch and store them in conjunction with the
+ * i_version.
+ *
  * Not all filesystems properly implement the i_version counter. Subsystems that
  * want to use i_version field on an inode should first check whether the
  * filesystem sets the SB_I_VERSION flag (usually via the IS_I_VERSION macro).


-- 
Jeff Layton <jlayton@xxxxxxxxxx>