Date:   Fri, 09 Sep 2022 08:47:17 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     Theodore Ts'o <tytso@....edu>
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, 2022-09-09 at 08:11 -0400, Theodore Ts'o wrote:
> On Thu, Sep 08, 2022 at 01:40:11PM -0400, Jeff Layton wrote:
> > 
> > Ted, how would we access this? Maybe we could just add a new (generic)
> > super_block field for this that ext4 (and other filesystems) could
> > populate at mount time?
> 
> Yeah, I was thinking about just adding it to struct super, with some
> value (perhaps 0 or ~0) meaning that the file system didn't support
> it.  If people were concerned about struct super bloat, we could also
> add some new function to struct super_ops that would return one or
> more values that are used rarely by most of the kernel code, and so
> doesn't need to be in the struct super data structure.  I don't have
> strong feelings one way or another.
> 

Either would be fine, I think.

> On another note, my personal opinion is that at least as far as ext4
> is concerned, i_version on disk's only use is for NFS's convenience,

Technically, IMA uses it too, but it needs the same behavior as NFSv4.

> and so I have absolutely no problem with changing how and when
> i_version gets updated modulo concerns about impacting performance.
> That's one of the reasons why being able to update i_version only
> lazily, so that if we had, say, some workload that was doing O_DIRECT
> writes followed by fdatasync(), there wouldn't be any obligation to
> flush the inode out to disk just because we had bumped i_version
> appeals to me.
> 

i_version only changes now if someone has queried it since it was last
changed. That makes a huge difference in performance. We can try to
optimize it further, but it probably wouldn't move the needle much under
real workloads.

> But aside from that, I don't consider when i_version gets updated on
> disk, especially what the semantics are after a crash, and if we need
> to change things so that NFS can be more performant, I'm happy to
> accomodate.  One of the reasons why we implemented the ext4 fast
> commit feature was to improve performance for NFS workloads.
> 
> I know some XFS developers have some concerns here, but I just wanted
> to make it be explicit that (a) I'm not aware of any users who are
> depending on the i_version on-disk semantics, and (b) if they are
> depending on something which as far as I'm concerned in an internal
> implementation detail, we've made no promises to them, and they can
> get to keep both pieces.  :-)  This is especially since up until now,
> there is no supported, portable userspace interface to make i_version
> available to userspace.
> 

Great! That's what I was hoping for with ext4. Would you be willing to
pick up these two patches for v6.1?

https://lore.kernel.org/linux-ext4/20220908172448.208585-3-jlayton@kernel.org/T/#u
https://lore.kernel.org/linux-ext4/20220908172448.208585-4-jlayton@kernel.org/T/#u

They should be able to go in independently of the rest of the series and
I don't forsee any big changes to them.

Thanks,
-- 
Jeff Layton <jlayton@...nel.org>

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