On 09/03/2015 02:35 PM, Kirill A. Shutemov wrote:
> On 64 bit system we have enough space in struct page to encode
> compound_dtor and compound_order with unsigned int.
>
> On x86-64 it leads to slightly smaller code size due usesage of plain
> MOV instead of MOVZX (zero-extended move) or similar effect.
>
> allyesconfig:
>
> text data bss dec hex filename
> 159520446 48146736 72196096 279863278 10ae5fee vmlinux.pre
> 159520382 48146736 72196096 279863214 10ae5fae vmlinux.post
>
> On other architectures without native support of 16-bit data types the
> difference can be bigger.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> include/linux/mm_types.h | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index ecaf3b1d0216..39b0db74ba5e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -150,8 +150,13 @@ struct page {
> /* First tail page of compound page */
> struct {
> unsigned long compound_head; /* If bit zero is set */
I'm indifferent to this change. But some comment here to explain would avoid git
blame to figure out why it's done?
> +#ifdef CONFIG_64BIT
> + unsigned int compound_dtor;
> + unsigned int compound_order;
> +#else
> unsigned short int compound_dtor;
> unsigned short int compound_order;
> +#endif
> };
>
> #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
>
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