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

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Message-ID: <20170817111005.asebivpywpeyevfq@gmail.com>
Date:   Thu, 17 Aug 2017 13:10:05 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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@...temov.name> wrote:

> On Thu, Aug 17, 2017 at 11:00:38AM +0200, Ingo Molnar wrote:
> > 
> > * 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.
> 
> This change increase vmlinux size for defconfig + X86_5LEVEL=y by ~4k or
> 0.01%.

Please add this info to one of the changelogs.

BTW., some of that might be won back via the later optimizations, right?

BTW, I think what matters most is the .text section size, while 'size' would also 
add exceptions/out-of-line sections too.

Here's a way to display the .text section only:

 triton:~/tip> printf "# vmlinux .text size: %d\n" 0x$(echo $(readelf -W -e vmlinux | grep ' \[ 1\] .text ') | cut -d' ' -f7)
 # vmlinux .text size: 9797610

 triton:~/tip> size vmlinux
   text    data     bss     dec     hex filename
 10748306        4875500  876544 16500350         fbc67e vmlinux

Thanks,

	Ingo