linux-kernel - Re: [PATCH 3/5] arm64/kernel: jump_label: switch to relative references

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Date:   Thu, 28 Jun 2018 10:17:57 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/5] arm64/kernel: jump_label: switch to relative
 references

On Wed, Jun 27, 2018 at 06:06:02PM +0200, Ard Biesheuvel wrote:
> On a randomly chosen distro kernel build for arm64, vmlinux.o shows the
> following sections, containing jump label entries, and the associated
> RELA relocation records, respectively:
> 
>   ...
>   [38088] __jump_table      PROGBITS         0000000000000000  00e19f30
>        000000000002ea10  0000000000000000  WA       0     0     8
>   [38089] .rela__jump_table RELA             0000000000000000  01fd8bb0
>        000000000008be30  0000000000000018   I      38178   38088     8
>   ...
> 
> In other words, we have 190 KB worth of 'struct jump_entry' instances,
> and 573 KB worth of RELA entries to relocate each entry's code, target
> and key members. This means the RELA section occupies 10% of the .init
> segment, and the two sections combined represent 5% of vmlinux's entire
> memory footprint.
> 
> So let's switch from 64-bit absolute references to 32-bit relative
> references: this reduces the size of the __jump_table by 50%, and gets
> rid of the RELA section entirely.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  arch/arm64/Kconfig                  |  1 +
>  arch/arm64/include/asm/jump_label.h | 16 ++++------------
>  arch/arm64/kernel/jump_label.c      |  6 +++---
>  3 files changed, 8 insertions(+), 15 deletions(-)

Looks good, cheers:

Acked-by: Will Deacon <will.deacon@....com>

Will