linux-kernel - Re: [PATCH v3] kallsyms: add support for relative offsets in kallsyms address table

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Message-Id: <20160122130718.8313ec9aefa654f33bd17792@linux-foundation.org>
Date:	Fri, 22 Jan 2016 13:07:18 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Michal Marek <mmarek@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v3] kallsyms: add support for relative offsets in
 kallsyms address table

On Fri, 22 Jan 2016 09:20:41 +0100 Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> > : in half since offsets can typically be expressed in 32 bits.
> > :
> """
> 
> In addition to fixing the broken grammar, would it make sense to
> mention that dynamic relocation only occurs under
> CONFIG_RELOCATABLE=y? I.e., something like
> 
> """
> On 64-bit architectures, it cuts the size of the kallsyms address
> table in half, since offsets between kernel symbols can typically be
> expressed in 32 bits. This saves several hundreds of kilobytes of
> permanent .rodata on average. In addition, the kallsyms address table
> is no longer subject to dynamic relocation when CONFIG_RELOCATABLE is
> in effect, so the relocation work done after decompression now doesn't
> have to do relocation updates for all these values. This saves up to
> 24 bytes (i.e., the size of a ELF64 RELA relocation table entry) per
> table entry, which easily adds up to a couple of megabytes of
> uncompressed __init data on ppc64 or arm64. Even if these relocation
> entries typically compress well, the combined size reduction of 2.8 MB
> uncompressed for a ppc64_defconfig build (of which 2.4 MB is __init
> data) results in a ~500 KB space saving in the compressed image.
> """

Yes, that sounds very good.  I'd buy one :)

Can you please send along a complete new changelog sometime?