linux-kernel - Re: [PATCH] mm: setup pageblock_order before it's used by sparse

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Date:	Mon, 2 Jul 2012 13:41:03 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Jiang Liu <jiang.liu@...wei.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Xishi Qiu <qiuxishi@...wei.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keping Chen <chenkeping@...wei.com>, linux-mm@...ck.org,
	stable@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiang Liu <liuj97@...il.com>
Subject: Re: [PATCH] mm: setup pageblock_order before it's used by sparse

On Sat, Jun 30, 2012 at 05:07:54PM +0800, Jiang Liu wrote:
> From: Xishi Qiu <qiuxishi@...wei.com>
> 
> On architectures with CONFIG_HUGETLB_PAGE_SIZE_VARIABLE set, such as Itanium,
> pageblock_order is a variable with default value of 0. It's set to the right
> value by set_pageblock_order() in function free_area_init_core().
> 
> But pageblock_order may be used by sparse_init() before free_area_init_core()
> is called along path:
> sparse_init()
>     ->sparse_early_usemaps_alloc_node()
> 	->usemap_size()
> 	    ->SECTION_BLOCKFLAGS_BITS
> 		->((1UL << (PFN_SECTION_SHIFT - pageblock_order)) *
> NR_PAGEBLOCK_BITS)
> 
> The uninitialized pageblock_size will cause memory wasting because usemap_size()
> returns a much bigger value then it's really needed.
> 
> For example, on an Itanium platform,
> sparse_init() pageblock_order=0 usemap_size=24576
> free_area_init_core() before pageblock_order=0, usemap_size=24576
> free_area_init_core() after pageblock_order=12, usemap_size=8
> 
> That means 24K memory has been wasted for each section, so fix it by calling
> set_pageblock_order() from sparse_init().
> 
> Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>
> Signed-off-by: Jiang Liu <liuj97@...il.com>

Looks all right.

Acked-by: Mel Gorman <mgorman@...e.de>

This should be considered a stable candidate. Add

Cc: stable <stable@...r.kernel.org>

above your Signed-off-by and it'll get picked up if the patch is merged
to mainline.

-- 
Mel Gorman
SUSE Labs
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