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

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Date:   Mon, 12 Sep 2022 08:59:25 -0400
From:   Jeff Layton <jlayton@...nel.org>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Theodore Ts'o <tytso@....edu>, 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 Mon, 2022-09-12 at 08:54 -0400, J. Bruce Fields wrote:
> On Mon, Sep 12, 2022 at 07:42:16AM -0400, Jeff Layton wrote:
> > A scheme like that could work. It might be hard to do it without a
> > spinlock or something, but maybe that's ok. Thinking more about how we'd
> > implement this in the underlying filesystems:
> > 
> > To do this we'd need 2 64-bit fields in the on-disk and in-memory 
> > superblocks for ext4, xfs and btrfs. On the first mount after a crash,
> > the filesystem would need to bump s_version_max by the significant
> > increment (2^40 bits or whatever). On a "clean" mount, it wouldn't need
> > to do that.
> > 
> > Would there be a way to ensure that the new s_version_max value has made
> > it to disk? Bumping it by a large value and hoping for the best might be
> > ok for most cases, but there are always outliers, so it might be
> > worthwhile to make an i_version increment wait on that if necessary. 
> 
> I was imagining that when you recognize you're getting close, you kick
> off something which writes s_version_max+2^40 to disk, and then updates
> s_version_max to that new value on success of the write.
> 

Ok, that makes sense.

> The code that increments i_version checks to make sure it wouldn't
> exceed s_version_max.  If it would, something has gone wrong--a write
> has failed or taken a long time--so it waits or errors out or something,
> depending on desired filesystem behavior in that case.
> 

Maybe could just throw a big scary pr_warn too? I'd have to think about
how we'd want to handle this case.

> No locking required in the normal case?

Yeah, maybe not.
-- 
Jeff Layton <jlayton@...nel.org>