linux-kernel - Re: [PATCHv4 08/14] x86/mm: Make PGDIR_SHIFT and PTRS_PER

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Date:   Thu, 17 Aug 2017 11:00:38 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 08/14] x86/mm: Make PGDIR_SHIFT and PTRS_PER_P4D
 variable


* Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:

> For boot-time switching between 4- and 5-level paging we need to be able
> to fold p4d page table level at runtime. It requires variable
> PGDIR_SHIFT and PTRS_PER_P4D.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/boot/compressed/kaslr.c        |  5 +++++
>  arch/x86/include/asm/pgtable_32.h       |  2 ++
>  arch/x86/include/asm/pgtable_32_types.h |  2 ++
>  arch/x86/include/asm/pgtable_64_types.h | 15 +++++++++++++--
>  arch/x86/kernel/head64.c                |  9 ++++++++-
>  arch/x86/mm/dump_pagetables.c           | 12 +++++-------
>  arch/x86/mm/init_64.c                   |  2 +-
>  arch/x86/mm/kasan_init_64.c             |  2 +-
>  arch/x86/platform/efi/efi_64.c          |  4 ++--
>  include/asm-generic/5level-fixup.h      |  1 +
>  include/asm-generic/pgtable-nop4d.h     |  1 +
>  include/linux/kasan.h                   |  2 +-
>  mm/kasan/kasan_init.c                   |  2 +-
>  13 files changed, 43 insertions(+), 16 deletions(-)

So I'm wondering what the code generation effect of this is - what's the 
before/after vmlinux size?

My guess is that the effect should be very small, as these constants are not 
widely used - but I'm only guessing and could be wrong.

Thanks,

	Ingo