linux-kernel - Re: [RFC PATCH 04/35] ceph: Convert ceph_mds_request::r

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Message-ID: <3188326.1742508102@warthog.procyon.org.uk>
Date: Thu, 20 Mar 2025 22:01:42 +0000
From: David Howells <dhowells@...hat.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>
Cc: dhowells@...hat.com, "slava@...eyko.com" <slava@...eyko.com>,
    "dongsheng.yang@...ystack.cn" <dongsheng.yang@...ystack.cn>,
    "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
    "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
    "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
    Alex Markuze <amarkuze@...hat.com>,
    "jlayton@...nel.org" <jlayton@...nel.org>,
    "idryomov@...il.com" <idryomov@...il.com>,
    "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: Re: [RFC PATCH 04/35] ceph: Convert ceph_mds_request::r_pagelist to a databuf

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

> > > > +	if (ceph_databuf_insert_frag(dbuf, 0, sizeof(*header),
> > > > GFP_KERNEL) < 0)
> > > > +		goto out;
> > > > +	if (ceph_databuf_insert_frag(dbuf, 1, PAGE_SIZE, GFP_KERNEL)
> > > > < 0)
> > > >  		goto out;
> > > >  
> > > > +	iov_iter_bvec(&iter, ITER_DEST, &dbuf->bvec[1], 1, len);
> > > 
> > > Is it correct &dbuf->bvec[1]? Why do we work with item #1? I think it
> > > looks confusing.
> > 
> > Because you have a protocol element (in dbuf->bvec[0]) and a buffer (in
> > dbuf->bvec[1]).
> 
> It sounds to me that we need to have two declarations (something like this):
> 
> #define PROTOCOL_ELEMENT_INDEX    0
> #define BUFFER_INDEX              1

But that's specific to this particular usage.  There may or may not be a
frag/page allocated to a protocol element and there may or may not be buffer
parts and may be multiple buffer parts.  There could even be multiple pages
allocated to protocol elements.

David