linux-kernel - Re: [RFC][PATCH 0/9] extend hugepage migration

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Date:	Sun, 07 Apr 2013 08:32:30 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Linux Memory Management List <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Linux kernel Mailing List <linux-kernel@...r.kernel.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [RFC][PATCH 0/9] extend hugepage migration

Hi Michal,
On 04/05/2013 05:30 PM, Michal Hocko wrote:
> On Fri 05-04-13 17:00:58, Simon Jeons wrote:
>> Hi Michal,
>> On 04/05/2013 04:08 PM, Michal Hocko wrote:
>>> On Fri 05-04-13 09:14:58, Simon Jeons wrote:
>>>> Hi Michal,
>>>> On 03/22/2013 04:15 PM, Michal Hocko wrote:
>>>>> [getting off-list]
>>>>>
>>>>> On Fri 22-03-13 07:46:32, Simon Jeons wrote:
>>>>>> Hi Michal,
>>>>>> On 03/21/2013 08:56 PM, Michal Hocko wrote:
>>>>>>> On Thu 21-03-13 07:49:48, Simon Jeons wrote:
>>>>>>> [...]
>>>>>>>> When I hacking arch/x86/mm/hugetlbpage.c like this,
>>>>>>>> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
>>>>>>>> index ae1aa71..87f34ee 100644
>>>>>>>> --- a/arch/x86/mm/hugetlbpage.c
>>>>>>>> +++ b/arch/x86/mm/hugetlbpage.c
>>>>>>>> @@ -354,14 +354,13 @@ hugetlb_get_unmapped_area(struct file *file,
>>>>>>>> unsigned long addr,
>>>>>>>>
>>>>>>>> #endif /*HAVE_ARCH_HUGETLB_UNMAPPED_AREA*/
>>>>>>>>
>>>>>>>> -#ifdef CONFIG_X86_64
>>>>>>>> static __init int setup_hugepagesz(char *opt)
>>>>>>>> {
>>>>>>>> unsigned long ps = memparse(opt, &opt);
>>>>>>>> if (ps == PMD_SIZE) {
>>>>>>>> hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT);
>>>>>>>> - } else if (ps == PUD_SIZE && cpu_has_gbpages) {
>>>>>>>> - hugetlb_add_hstate(PUD_SHIFT - PAGE_SHIFT);
>>>>>>>> + } else if (ps == PUD_SIZE) {
>>>>>>>> + hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT+4);
>>>>>>>> } else {
>>>>>>>> printk(KERN_ERR "hugepagesz: Unsupported page size %lu M\n",
>>>>>>>> ps >> 20);
>>>>>>>>
>>>>>>>> I set boot=hugepagesz=1G hugepages=10, then I got 10 32MB huge pages.
>>>>>>>> What's the difference between these pages which I hacking and normal
>>>>>>>> huge pages?
>>>>>>> How is this related to the patch set?
>>>>>>> Please _stop_ distracting discussion to unrelated topics!
>>>>>>>
>>>>>>> Nothing personal but this is just wasting our time.
>>>>>> Sorry kindly Michal, my bad.
>>>>>> Btw, could you explain this question for me? very sorry waste your time.
>>>>> Your CPU has to support GB pages. You have removed cpu_has_gbpages test
>>>>> and added a hstate for order 13 pages which is a weird number on its
>>>>> own (32MB) because there is no page table level to support them.
>>>> But after hacking, there is /sys/kernel/mm/hugepages/hugepages-*,
>>>> and have equal number of 32MB huge pages which I set up in boot
>>>> parameter.
>>> because hugetlb_add_hstate creates hstate for those pages and
>>> hugetlb_init_hstates allocates them later on.
>>>
>>>> If there is no page table level to support them, how can
>>>> them present?
>>> Because hugetlb hstate handling code doesn't care about page tables and
>>> the way how those pages are going to be mapped _at all_. Or put it in
>>> another way. Nobody prevents you to allocate order-5 page for a single
>>> pte but that would be a pure waste. Page fault code expects that pages
>>> with a proper size are allocated.
>> Do you mean 32MB pages will map to one pmd which should map 2MB pages?
>>
> Please refer to hugetlb_fault for more information.

Thanks for your pointing out. So my assume is correct, is it? Can pmd 
which support 2MB map 32MB pages work well?

>

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