linux-kernel - Re: [PATCHv5 7/7] mm: use 'unsigned int' for compound_dtor/compound

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Message-ID: <55F16ACD.2010104@suse.cz>
Date:	Thu, 10 Sep 2015 13:34:37 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 7/7] mm: use 'unsigned int' for
 compound_dtor/compound_order on 64BIT

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@...ux.intel.com>
> Acked-by: Michal Hocko <mhocko@...e.com>
> ---
>  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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