linux-kernel - Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount

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Date:   Fri, 17 Jun 2022 14:06:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>
Subject: Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

On Fri, Jun 17, 2022 at 11:44:49AM +0000, Christophe Leroy wrote:
> 
> 
> Le 17/06/2022 à 13:24, Peter Zijlstra a écrit :
> > 
> > Hi,
> > 
> > I recently noticed that __mcount_loc is 64bit wide, containing absolute
> > addresses. Since __mcount_loc is a permanent section (not one we drop
> > after boot), this bloats the kernel memory usage for no real purpose.
> 
> I guess you mean it is 64bit wide on 64 bits architectures ? Because it 
> seems to be defined as 'long' so will be 32bit on 32 bits architectures ?

Right, it's an absolute address in the native size. Clearly I'm somewhat
64bit biassed :-)