linux-kernel - Re: [PATCHv2 3/4] mm: pack compound_dtor and compound

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Message-ID: <20150818154259.GL5033@dhcp22.suse.cz>
Date:	Tue, 18 Aug 2015 17:43:00 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 3/4] mm: pack compound_dtor and compound_order into one
 word in struct page

On Mon 17-08-15 18:09:04, Kirill A. Shutemov 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Reviewed-by: Michal Hocko <mhocko@...e.com>

[...]
> @@ -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 do we need this ifdef? We can go with short for both 32b and 64b
AFAICS. We do not use compound_order for anything else than the order,
right?

While I am looking at this, it seems we are jugling with type for order
quite a lot - int, unsing int and even unsigned long.
-- 
Michal Hocko
SUSE Labs
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