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

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Date:   Fri, 24 Jun 2022 09:16:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        christophe.leroy@...roup.eu, naveen.n.rao@...ux.vnet.ibm.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

On Wed, Jun 22, 2022 at 10:50:18AM -0400, Steven Rostedt wrote:
> On Fri, 17 Jun 2022 13:24:53 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > 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.
> 
> Wait, it's not dropped? Nothing uses it after it is read. It should be
> dropped when init data is dropped.
> 
> From include/asm-generic/vmlinux.lds.h
> 
> /* init and exit section handling */
> #define INIT_DATA                                                       \
>         KEEP(*(SORT(___kentry+*)))                                      \
>         *(.init.data init.data.*)                                       \
>         MEM_DISCARD(init.data*)                                         \
>         KERNEL_CTORS()                                                  \
>         MCOUNT_REC()                                                    \  <<----
>         *(.init.rodata .init.rodata.*)                                  \
>         FTRACE_EVENTS()                                                 \
>         TRACE_SYSCALLS()                                                \
>         KPROBE_BLACKLIST()                                              \
>         ERROR_INJECT_WHITELIST()                                        \
>         MEM_DISCARD(init.rodata)                                        \
> 
> 
> So it should be dropped after boot.

Urgh, clearly I got lost somewhere along the line. In that case it
doesn't matter nearly as much.