linux-ext4 - Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing

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Date:   Tue, 31 Oct 2023 11:26:22 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.de>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing

On Thu, Oct 19, 2023 at 07:28:48AM -0400, Jeff Layton wrote:
> On Thu, 2023-10-19 at 11:29 +0200, Christian Brauner wrote:
> > > Back to your earlier point though:
> > > 
> > > Is a global offset really a non-starter? I can see about doing something
> > > per-superblock, but ktime_get_mg_coarse_ts64 should be roughly as cheap
> > > as ktime_get_coarse_ts64. I don't see the downside there for the non-
> > > multigrain filesystems to call that.
> > 
> > I have to say that this doesn't excite me. This whole thing feels a bit
> > hackish. I think that a change version is the way more sane way to go.
> > 
> 
> What is it about this set that feels so much more hackish to you? Most
> of this set is pretty similar to what we had to revert. Is it just the
> timekeeper changes? Why do you feel those are a problem?

So I think that the multi-grain timestamp work was well intended but it
was ultimately a mistake. Because we added code that complicated
timestamp timestamp handling in the vfs to a point where the costs
clearly outweighed the benefits.

And I don't think that this direction is worth going into. This whole
thread ultimately boils down to complicating generic infrastructure
quite extensively for nfs to handle exposing xfs without forcing an
on-disk format change. That's even fine.

That's not a problem but in the same way I don't think the solution is
just stuffing this complexity into the vfs. IOW, if we make this a vfs
problem then at the lowest possible cost and not by changing how
timestamps work for everyone even if it's just internal.