Received-SPF: pass (google.com: domain of linux-ext4-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 9 Sep 2022 10:58:00 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     "J. Bruce Fields" <bfields@fieldses.org>, Jan Kara <jack@suse.cz>,
        NeilBrown <neilb@suse.de>, adilger.kernel@dilger.ca,
        djwong@kernel.org, david@fromorbit.com, trondmy@hammerspace.com,
        viro@zeniv.linux.org.uk, zohar@linux.ibm.com, xiubli@redhat.com,
        chuck.lever@oracle.com, lczerner@redhat.com, brauner@kernel.org,
        fweimer@redhat.com, linux-man@vger.kernel.org,
        linux-api@vger.kernel.org, linux-btrfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        ceph-devel@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-nfs@vger.kernel.org, linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field
Message-ID: <YxtUeE4OouV7jUF9@mit.edu>
References: <166259786233.30452.5417306132987966849@noble.neil.brown.name>
 <20220908083326.3xsanzk7hy3ff4qs@quack3>
 <YxoIjV50xXKiLdL9@mit.edu>
 <02928a8c5718590bea5739b13d6b6ebe66cac577.camel@kernel.org>
 <20220908155605.GD8951@fieldses.org>
 <9e06c506fd6b3e3118da0ec24276e85ea3ee45a1.camel@kernel.org>
 <YxstWiu34TfJ6muW@mit.edu>
 <6173b33e43ac8b0e4377b5d65fec7231608f71f7.camel@kernel.org>
 <YxtEHIkfX0nQQC0n@mit.edu>
 <8b556c2dadb717a25ab47f02f70cfaaa6c6074c7.camel@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <8b556c2dadb717a25ab47f02f70cfaaa6c6074c7.camel@kernel.org>
Precedence: bulk

On Fri, Sep 09, 2022 at 10:43:30AM -0400, Jeff Layton wrote:

> In general, we want to bump i_version if the ctime changes. I'm guessing
> that we don't change ctime on a delalloc? If it's not visible to NFS,
> then NFS won't care about it.  We can't project FIEMAP info across the
> wire at this time, so we'd probably like to avoid seeing an i_version
> bump in due to delalloc.

Right, currently nothing user-visible changes when delayed allocation
is resolved; ctime isn't bumped, and i_version shouldn't be bumped
either.

If we crash before delayed allocation is resolved, there might be
cases (mounting with data=writeback is the one which I'm most worried
about, but I haven't experimented to be sure) where the inode might
become a zero-length file after the reboot without i_version or ctime
changing, but given that NFS forces a fsync(2) before it acknowledges
a client request, that shouldn't be an issue for NFS.

This is where as far I'm concerned, for ext4, i_version has only one
customer to keep happy, and it's NFS.  :-)    Now, if we expose i_version
via statx(2), we might need to be a tad bit more careful about what
semantics we guarantee to userspace, especially with respect to what
might be returned before and after a crash recovery.  If we can leave
things such that there is maximal freedom for file system
implementations, that would be my preference.

						- Ted