linux-ext4 - Re: [PATCH v8 RESEND 2/8] fs: clarify when the i

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Message-ID: <20230126113642.eenghs2wvfrlnlak@quack3>
Date:   Thu, 26 Jan 2023 12:36:42 +0100
From:   Jan Kara <jack@...e.cz>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, tytso@....edu, adilger.kernel@...ger.ca,
        djwong@...nel.org, david@...morbit.com, trondmy@...merspace.com,
        neilb@...e.de, viro@...iv.linux.org.uk, zohar@...ux.ibm.com,
        xiubli@...hat.com, chuck.lever@...cle.com, lczerner@...hat.com,
        bfields@...ldses.org, brauner@...nel.org, fweimer@...hat.com,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org, Colin Walters <walters@...bum.org>
Subject: Re: [PATCH v8 RESEND 2/8] fs: clarify when the i_version counter
 must be updated

On Thu 26-01-23 05:54:16, Jeff Layton wrote:
> On Wed, 2023-01-25 at 17:06 +0100, Jan Kara wrote:
> > On Tue 24-01-23 14:30:19, Jeff Layton wrote:
> > > The i_version field in the kernel has had different semantics over
> > > the decades, but NFSv4 has certain expectations. Update the comments
> > > in iversion.h to describe when the i_version must change.
> > > 
> > > Cc: Colin Walters <walters@...bum.org>
> > > Cc: NeilBrown <neilb@...e.de>
> > > Cc: Trond Myklebust <trondmy@...merspace.com>
> > > Cc: Dave Chinner <david@...morbit.com>
> > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > 
> > Looks good to me. But one note below:
> > 
> > > diff --git a/include/linux/iversion.h b/include/linux/iversion.h
> > > index 6755d8b4f20b..fced8115a5f4 100644
> > > --- a/include/linux/iversion.h
> > > +++ b/include/linux/iversion.h
> > > @@ -9,8 +9,25 @@
> > >   * ---------------------------
> > >   * 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 larger to observers if there was an explicit change to the inode's
> > > + * data or metadata since it was last queried.
> > > + *
> > > + * An explicit change is one that would ordinarily result in a change to the
> > > + * inode status change time (aka ctime). i_version must appear to change, even
> > > + * if the ctime does not (since the whole point is to avoid missing updates due
> > > + * to timestamp granularity). If POSIX or other relevant spec mandates that the
> > > + * ctime must change due to an operation, then the i_version counter must be
> > > + * incremented as well.
> > > + *
> > > + * Making the i_version update completely atomic with the operation itself would
> > > + * be prohibitively expensive. Traditionally the kernel has updated the times on
> > > + * directories after an operation that changes its contents. For regular files,
> > > + * the ctime is usually updated before the data is copied into the cache for a
> > > + * write. This means that there is a window of time when an observer can
> > > + * associate a new timestamp with old file contents. Since the purpose of the
> > > + * i_version is to allow for better cache coherency, the i_version must always
> > > + * be updated after the results of the operation are visible. Updating it before
> > > + * and after a change is also permitted.
> > 
> > This sounds good but it is not the case for any of the current filesystems, is
> > it? Perhaps the documentation should mention this so that people are not
> > confused?
> 
> Correct. Currently, all filesystems change the times and version before
> a write instead of after. I'm hoping that situation will change soon
> though, as I've been working on a patchset to fix this for tmpfs, ext4
> and btrfs.

That is good but we'll see how long it takes to get merged. AFAIR it is not
a complete nobrainer ;)

> If you still want to see something for this though, what would you
> suggest for verbiage?

Sure:

... the i_version must a be updated after the results of the operation are
visible (note that none of the filesystems currently do this, it is a work
in progress to fix this).

And once your patches are merged, you can also delete this note :).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR