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SubjectRe: Re: [PATCH] Remove initialization of page_table in one_page_table_init()

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

> Ingo Molnar wrote:
> > * Zhaolei <zhaolei@cn.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@cn.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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