linux-kernel - Re: [PATCH] mm: sparse: use __aligned() instead of manual padding in mem

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Date:	Thu, 30 May 2013 08:54:35 +0800
From:	Jiang Liu <liuj97@...il.com>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: sparse: use __aligned() instead of manual padding
 in mem_section

On Thu 30 May 2013 07:14:39 AM CST, Cody P Schafer wrote:
> Instead of leaving a trap for the next person who comes along and wants
> to add something to mem_section, add an __aligned() and remove the
> manual padding added for MEMCG.
>
> Signed-off-by: Cody P Schafer <cody@...ux.vnet.ibm.com>
> ---
>  include/linux/mmzone.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> ---
>
> Also, does anyone know what causes this alignment to be required here? I found
> this was breaking things in a patchset I'm working on (WARNs in sysfs code
> about duplicate filenames when initing mem_sections). Adding some documentation
> for the reason would be appreciated.
Hi Cody,
        I think the alignment requirement is caused by the way the 
mem_section array is
organized. Basically it requires that PAGE_SIZE could be divided by 
sizeof(struct mem_section).
So your change seems risky too because it should be aligned to power of 
two instead
of 2 * sizeof(long).
Regards!
Gerry

>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 131989a..a8e8056 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1125,9 +1125,8 @@ struct mem_section {
>  	 * section. (see memcontrol.h/page_cgroup.h about this.)
>  	 */
>  	struct page_cgroup *page_cgroup;
> -	unsigned long pad;
>  #endif
> -};
> +} __aligned(2 * sizeof(unsigned long));
>
>  #ifdef CONFIG_SPARSEMEM_EXTREME
>  #define SECTIONS_PER_ROOT       (PAGE_SIZE / sizeof (struct mem_section))


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