[PATCH bpf-next 0/1] bpf: btf: make vmlinux BTF disposable at boot to save ~5 MiB

[Jay Wang]

Based on and tested against mainline commit 9b87fdc9af2f ("Merge tag
'sched_ext-for-7.3-rc3-fixes' of
git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext").

This patch adds a "btf=off" boot parameter that disables the vmlinux BTF
and frees the pages backing it, recovering ~5 MiB.  The reasons for making
it a boot-time choice are below.

A distribution ships one kernel binary to every user, so each CONFIG_*
choice applies to all of them.  CONFIG_DEBUG_INFO_BTF is one where that
hurts: the vmlinux .BTF section stays in rodata for the whole boot,
5.2 MiB on x86-64 and 5.3 MiB on arm64 with a distribution config.  On a
1 GiB VM, a common cloud instance size, that is a visible part of total
memory.  Off takes BTF away from the users who need it; on charges the
users who never touch it.

Letting the user decide at boot follows the existing
CONFIG_SECURITY_SELINUX_BOOTPARAM, which does the same for selinux=0.
With btf=off the kernel behaves as if CONFIG_DEBUG_INFO_BTF were not set,
so nothing can reach the section and its pages are freed.  BPF features
needing kernel type information then fail program load cleanly, exactly as
on a config-off kernel.  By default the parameter is not set and nothing
changes.

The freeing implementation is architecture-specific, and btf=off is
currently supported on x86-64 and arm64.

Testing: x86-64 and arm64 boot with and without btf=off.  With btf=off,
/sys/kernel/btf is absent and ~5 MiB is returned to the page allocator.

Jay Wang (1):
  bpf: btf: add btf=off boot parameter to disable and free vmlinux BTF

 .../admin-guide/kernel-parameters.txt         |  14 +++
 arch/arm64/mm/mmu.c                           | 108 +++++++++++++++++-
 arch/x86/mm/init_64.c                         |  30 +++++
 arch/x86/mm/pat/set_memory.c                  |  28 ++++-
 include/linux/btf.h                           |   9 ++
 kernel/bpf/btf.c                              |  51 ++++++++-
 kernel/bpf/sysfs_btf.c                        |   7 ++
 kernel/bpf/verifier.c                         |   2 +-
 8 files changed, 241 insertions(+), 8 deletions(-)


base-commit: 9b87fdc9af2fbfcdb5c24a64139685ef80f6573f
-- 
2.47.3