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

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Date:   Mon, 21 Aug 2017 18:23:09 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ingo Molnar <mingo@...nel.org>
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

On Thu, Aug 17, 2017 at 01:10:05PM +0200, Ingo Molnar wrote:
> 
> * 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?

Sorry, but no. Just chacked. Introducing alternatives increases .text
segment size by about 4k.

-- 
 Kirill A. Shutemov