linux-kernel - Re: [PATCHv7 11/19] x86/mm: Make STACK_TOP

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Message-ID: <20170928131910.7t7ops6b7h7fcrmm@node.shutemov.name>
Date:   Thu, 28 Sep 2017 16:19:10 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 11/19] x86/mm: Make STACK_TOP_MAX dynamic

On Thu, Sep 28, 2017 at 10:29:55AM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> 
> > For boot-time switching between paging modes, we need to be able to
> > change STACK_TOP_MAX at runtime.
> > 
> > The change is trivial and it doesn't affect kernel image size.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/include/asm/processor.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > index 3fa26a61eabc..fa9300ccce1b 100644
> > --- a/arch/x86/include/asm/processor.h
> > +++ b/arch/x86/include/asm/processor.h
> > @@ -871,7 +871,7 @@ static inline void spin_lock_prefetch(const void *x)
> >  					IA32_PAGE_OFFSET : TASK_SIZE_MAX)
> >  
> >  #define STACK_TOP		TASK_SIZE_LOW
> > -#define STACK_TOP_MAX		TASK_SIZE_MAX
> > +#define STACK_TOP_MAX		(pgtable_l5_enabled ? TASK_SIZE_MAX : DEFAULT_MAP_WINDOW)
> 
> While it's only used once in fs/exec.c, why doesn't it affect kernel image size?

Oh. After closer look the patch is redundant. The STACK_TOP_MAX is already
dynamic due to dynamic TASK_SIZE_MAX, so gcc generates exactly the same
code before and after the patch.

I'll drop it.

-- 
 Kirill A. Shutemov