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Date: Tue, 3 Jul 2012 11:29:42 +0800
From: Jiang Liu <jiang.liu@huawei.com>
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To: Yinghai Lu <yinghai@kernel.org>
Subject: Re: [PATCH] mm: setup pageblock_order before it's used by sparse
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Cc: Tony Luck <tony.luck@intel.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, stable@vger.kernel.org,
 Minchan Kim <minchan@kernel.org>, Keping Chen <chenkeping@huawei.com>,
 Mel Gorman <mgorman@suse.de>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
 David Rientjes <rientjes@google.com>, Xishi Qiu <qiuxishi@huawei.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 David Gibson <david@gibson.dropbear.id.au>, linuxppc-dev@lists.ozlabs.org,
 KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
 Jiang Liu <liuj97@gmail.com>
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>> Hi Yinghai,
>>
>> I'm afraid the v2 will break powerpc. Currently only IA64 and PowerPC
>> supports variable hugetlb size.
>>
>> HPAGE_SHIFT is a variable default to 0 on powerpc. But seems PowerPC
>> is doing something wrong here, according to it's mm initialization
>> sequence as below:
>> start_kernel()
>>         setup_arch()
>>                 paging_init()
>>                         free_area_init_node()
>>                                 set_pageblock_order()
>>                                         refer to HPAGE_SHIFT (still 0)
>>         init_rest()
>>                 do_initcalls()
>>                         hugetlbpage_init()
>>                                 setup HPAGE_SHIFT
>> That means pageblock_order is always set to "MAX_ORDER - 1", not sure
>> whether this is intended. And it has the same issue as IA64 of wasting
>> memory if CONFIG_SPARSE is enabled.
> 
> adding BenH, need to know if it is powerpc intended.
> 
>>
>> So it would be better to keep function set_pageblock_order(), it will
>> fix the memory wasting on both IA64 and PowerPC.
> 
> Should setup pageblock_order as early as possible to avoid confusing.
OK, waiting response from PPC. If we could find some ways to set HPAGE_SIZE
early on PPC too, we can setup pageblock_order in arch instead of page_alloc.c
as early as possible.

Thanks!
Gerry