linux-ext4 - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Date:   Fri, 9 Sep 2022 10:58:00 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>, Jan Kara <jack@...e.cz>,
        NeilBrown <neilb@...e.de>, adilger.kernel@...ger.ca,
        djwong@...nel.org, david@...morbit.com, trondmy@...merspace.com,
        viro@...iv.linux.org.uk, zohar@...ux.ibm.com, xiubli@...hat.com,
        chuck.lever@...cle.com, lczerner@...hat.com, brauner@...nel.org,
        fweimer@...hat.com, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, 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
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

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