linux-kernel - Re: [RFC PATCH 19/35] libceph, ceph: Convert users of ceph_pagelist to ceph

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Message-ID: <2680947.1742336824@warthog.procyon.org.uk>
Date: Tue, 18 Mar 2025 22:27:04 +0000
From: David Howells <dhowells@...hat.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....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>,
    "idryomov@...il.com" <idryomov@...il.com>,
    "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: Re: [RFC PATCH 19/35] libceph, ceph: Convert users of ceph_pagelist to ceph_databuf

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

> >  		/*
> > -		 * number of encoded locks is stable, so copy to pagelist
> > +		 * number of encoded locks is stable, so copy to databuf
> >  		 */
> >  		struct_len = 2 * sizeof(u32) +
> >  			    (num_fcntl_locks + num_flock_locks) *
> 
> I think we have too many mysterious equations in CephFS code. :)

That's not particularly a function of these patches.

> > -		err = ceph_pagelist_reserve(pagelist,
> > -					    sizeof(u64) + sizeof(u32) +
> > -					    pathlen + sizeof(rec.v1));
> > +		err = ceph_databuf_reserve(dbuf,
> > +					   sizeof(u64) + sizeof(u32) +
> > +					   pathlen + sizeof(rec.v1),
> > +					   GFP_NOFS);
> 
> Yeah, another mysterious calculation. Why do we add sizeof(u64) and
> sizeof(u32) here?

Protocol element space.

David