linux-kernel - Re: Re: [PATCH] Remove initialization of page_table in one_page_table

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Message-ID: <20090720115003.GB29811@elte.hu>
Date:	Mon, 20 Jul 2009 13:50:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Zhaolei <zhaolei@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] Remove initialization of page_table in
	one_page_table_init()


* Zhaolei <zhaolei@...fujitsu.com> wrote:

> Ingo Molnar wrote:
> > * Zhaolei <zhaolei@...fujitsu.com> wrote:
> > 
> >> Only judge if (!page_table) when CONFIG_DEBUG_PAGEALLOC or CONFIG_KMEMCHECK
> >> are defined, then we don't need to set init value for page_table.
> >>
> >> Signed-off-by: Zhao Lei <zhaolei@...fujitsu.com>
> >> ---
> >> arch/x86/mm/init_32.c |    4 ++--
> >> 1 files changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> >> index 3cd7711..55da09b 100644
> >> --- a/arch/x86/mm/init_32.c
> >> +++ b/arch/x86/mm/init_32.c
> >> @@ -108,13 +108,13 @@ static pmd_t * __init one_md_table_init(pgd_t *pgd)
> >> static pte_t * __init one_page_table_init(pmd_t *pmd)
> >> {
> >>     if (!(pmd_val(*pmd) & _PAGE_PRESENT)) {
> >> -        pte_t *page_table = NULL;
> >> +        pte_t *page_table;
> >>
> >>         if (after_bootmem) {
> >> #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_KMEMCHECK)
> >>             page_table = (pte_t *) alloc_bootmem_pages(PAGE_SIZE);
> >> -#endif
> >>             if (!page_table)
> >> +#endif
> >>                 page_table =
> >>                 (pte_t *)alloc_bootmem_low_pages(PAGE_SIZE);
> > 
> > I think the original code was cleaner - putting an #ifdef straight 
> > into the middle of a control block is quite fragile.
> > 
> > 	Ingo
> > 
> 
> Hello, Ingo
> 
> David Rientjes said that:
> Nack, gcc will already optimize the branch out for configs without 
> CONFIG_DEBUG_PAGEALLOC and CONFIG_KMEMCHECK.  The only thing this change 
> does is make the code less readable.
> 
> I tested it with gcc -O2, and proved that David is correct.
> Initialization of page_table and additional "if (!page_table)" was 
> removed by optimization. Please ignore this patch.

Yeah - and note that even if GCC _didnt_ optimize it out, the patch 
was not acceptable in this form.

	Ingo
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