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

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Date:	Mon, 2 Jul 2012 20:25:00 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Jiang Liu <jiang.liu@...wei.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Tony Luck <tony.luck@...el.com>,
	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>,
	David Gibson <david@...son.dropbear.id.au>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] mm: setup pageblock_order before it's used by sparse

On Mon, Jul 2, 2012 at 7:54 PM, Jiang Liu <jiang.liu@...wei.com> wrote:
> On 2012-7-3 4:43, Yinghai Lu wrote:
>> On Sun, Jul 1, 2012 at 7:01 PM, Jiang Liu <jiang.liu@...wei.com> wrote:
>>> Hi Yinghai,
>>>         The patch fails compilation as below:
>>> mm/page_alloc.c:151: error: initializer element is not constant
>>> mm/page_alloc.c:151: error: expected ‘,’ or ‘;’ before ‘__attribute__’
>>>
>>> On IA64, HUGETLB_PAGE_ORDER has dependency on variable hpage_shift.
>>> # define HUGETLB_PAGE_ORDER        (HPAGE_SHIFT - PAGE_SHIFT)
>>> # define HPAGE_SHIFT               hpage_shift
>>>
>>> And hpage_shift could be changed by early parameter "hugepagesz".
>>> So seems will still need to keep function set_pageblock_order().
>>
>> ah,  then use use _DEFAULT instead and later could update that in earlyparam.
>>
>> So attached -v2 should  work.
> 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.

Thanks

Yinghai
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