linux-kernel - Why use plain numbers and totals rather than predef'd constants for RPC sizes?

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Message-ID: <3172946.1742482785@warthog.procyon.org.uk>
Date: Thu, 20 Mar 2025 14:59:45 +0000
From: David Howells <dhowells@...hat.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>,
    Ilya Dryomov <idryomov@...il.com>
Cc: dhowells@...hat.com, Alex Markuze <amarkuze@...hat.com>,
    "slava@...eyko.com" <slava@...eyko.com>,
    "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
    "jlayton@...nel.org" <jlayton@...nel.org>,
    "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
    "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
    "dongsheng.yang@...ystack.cn" <dongsheng.yang@...ystack.cn>,
    "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Why use plain numbers and totals rather than predef'd constants for RPC sizes?

Viacheslav Dubeyko <Slava.Dubeyko@....com> wrote:

> > -	dbuf = ceph_databuf_reply_alloc(1, 8 + sizeof(struct ceph_timespec), GFP_NOIO);
> > -	if (!dbuf)
> > +	request = ceph_databuf_reply_alloc(1, 8 + sizeof(struct ceph_timespec), GFP_NOIO);
> 
> Ditto. Why do we have 8 + sizeof(struct ceph_timespec) here?

Because that's the size of the composite protocol element.

As to why it's using a total of plain integers and sizeofs rather than
constant macros, Ilya is the person to ask according to git blame;-).

I would probably prefer sizeof(__le64) here over 8, but I didn't want to
change it too far from the existing code.

If you want macro constants for these sorts of things, someone else who knows
the protocol better needs to do that.  You could probably write something to
generate them (akin to rpcgen).

David