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Date: Thu, 19 Oct 2023 11:29:11 +0200
From: Christian Brauner <brauner@kernel.org>
To: Jeff Layton <jlayton@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	John Stultz <jstultz@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Stephen Boyd <sboyd@kernel.org>,
	Chandan Babu R <chandan.babu@oracle.com>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Dave Chinner <david@fromorbit.com>, Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>, Chris Mason <clm@fb.com>,
	Josef Bacik <josef@toxicpanda.com>, David Sterba <dsterba@suse.com>,
	Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.de>,
	David Howells <dhowells@redhat.com>, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-mm@kvack.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain
 timestamp handing
Message-ID: <20231019-fluor-skifahren-ec74ceb6c63e@brauner>
References: <20231018-mgtime-v1-0-4a7a97b1f482@kernel.org>
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 <d6162230b83359d3ed1ee706cc1cb6eacfb12a4f.camel@kernel.org>
 <CAHk-=wiKJgOg_3z21Sy9bu+3i_34S86r8fd6ngvJpZDwa-ww8Q@mail.gmail.com>
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In-Reply-To: <5f96e69d438ab96099bb67d16b77583c99911caa.camel@kernel.org>

> 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.

> 
> On another note: maybe I need to put this behind a Kconfig option
> initially too?

So can we for a second consider not introducing fine-grained timestamps
at all. We let NFSv3 live with the cache problem it's been living with
forever.

And for NFSv4 we actually do introduce a proper i_version for all
filesystems that matter to it.

What filesystems exactly don't expose a proper i_version and what does
prevent them from adding one or fixing it?