linux-kernel - Re: [PATCHv3 3/5] mm: pack compound_dtor and compound

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Message-Id: <20150820162604.1a1dbbfeafefcda4327587af@linux-foundation.org>
Date:	Thu, 20 Aug 2015 16:26:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 3/5] mm: pack compound_dtor and compound_order into
 one word in struct page

On Wed, 19 Aug 2015 12:21:44 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> The patch halves space occupied by compound_dtor and compound_order in
> struct page.
> 
> For compound_order, it's trivial long -> int/short conversion.
> 
> For get_compound_page_dtor(), we now use hardcoded table for destructor
> lookup and store its index in the struct page instead of direct pointer
> to destructor. It shouldn't be a big trouble to maintain the table: we
> have only two destructor and NULL currently.
> 
> This patch free up one word in tail pages for reuse. This is preparation
> for the next patch.
> 
> ...
>
> @@ -145,8 +143,13 @@ struct page {
>  						 */
>  		/* First tail page of compound page */
>  		struct {
> -			compound_page_dtor *compound_dtor;
> -			unsigned long compound_order;
> +#ifdef CONFIG_64BIT
> +			unsigned int compound_dtor;
> +			unsigned int compound_order;
> +#else
> +			unsigned short int compound_dtor;
> +			unsigned short int compound_order;
> +#endif

Why not use ushort for 64-bit as well?

It would be clearer if that new enum had a name, so we use "enum
compound_dtor_id" everywhere instead of a bare uint.

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