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 <redacted>
---
 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 <redacted>

Will