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

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Date:   Mon, 12 Sep 2022 10:50:57 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "jlayton@...nel.org" <jlayton@...nel.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
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        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
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        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Mon, Sep 12, 2022 at 02:15:16PM +0000, Trond Myklebust wrote:
> On Mon, 2022-09-12 at 09:51 -0400, J. Bruce Fields wrote:
> > On Mon, Sep 12, 2022 at 08:55:04AM -0400, Jeff Layton wrote:
> > > Because of the "seen" flag, we have a 63 bit counter to play with.
> > > Could
> > > we use a similar scheme to the one we use to handle when "jiffies"
> > > wraps? Assume that we'd never compare two values that were more
> > > than
> > > 2^62 apart? We could add i_version_before/i_version_after macros to
> > > make
> > > it simple to handle this.
> > 
> > As far as I recall the protocol just assumes it can never wrap.  I
> > guess
> > you could add a new change_attr_type that works the way you describe.
> > But without some new protocol clients aren't going to know what to do
> > with a change attribute that wraps.
> > 
> > I think this just needs to be designed so that wrapping is impossible
> > in
> > any realistic scenario.  I feel like that's doable?
> > 
> > If we feel we have to catch that case, the only 100% correct behavior
> > would probably be to make the filesystem readonly.
> > 
> 
> Which protocol? If you're talking about basic NFSv4, it doesn't assume
> anything about the change attribute and wrapping.
> 
> The NFSv4.2 protocol did introduce the optional attribute
> 'change_attr_type' that tries to describe the change attribute
> behaviour to the client. It tells you if the behaviour is monotonically
> increasing, but doesn't say anything about the behaviour when the
> attribute value overflows.
> 
> That said, the Linux NFSv4.2 client, which uses that change_attr_type
> attribute does deal with overflow by assuming standard uint64_t wrap
> around rules. i.e. it assumes bit values > 63 are truncated, meaning
> that the value obtained by incrementing (2^64-1) is 0.

Yeah, it was the MONOTONIC_INCRE case I was thinking of.  That's
interesting, I didn't know the client did that.

--b.