linux-kernel - Re: [PATCH 1/4] pagemap: avoid splitting thp when reading /proc/pid/pagemap

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Date:	Tue, 03 Jan 2012 16:31:11 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	kosaki.motohiro@...il.com, linux-mm@...ck.org,
	akpm@...ux-foundation.org, rientjes@...gle.com,
	andi@...stfloor.org, fengguang.wu@...el.com, aarcange@...hat.com,
	kamezawa.hiroyu@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] pagemap: avoid splitting thp when reading /proc/pid/pagemap

(1/3/2012 16:06), KOSAKI Motohiro wrote:
> On 1/3/2012 3:07 PM, Naoya Horiguchi wrote:
>> On Thu, Dec 29, 2011 at 10:39:18PM -0500, KOSAKI Motohiro wrote:
>> ...
>>>> --- 3.2-rc5.orig/fs/proc/task_mmu.c
>>>> +++ 3.2-rc5/fs/proc/task_mmu.c
>>>> @@ -600,6 +600,9 @@ struct pagemapread {
>>>>   	u64 *buffer;
>>>>   };
>>>>
>>>> +#define PAGEMAP_WALK_SIZE	(PMD_SIZE)
>>>> +#define PAGEMAP_WALK_MASK	(PMD_MASK)
>>>> +
>>>>   #define PM_ENTRY_BYTES      sizeof(u64)
>>>>   #define PM_STATUS_BITS      3
>>>>   #define PM_STATUS_OFFSET    (64 - PM_STATUS_BITS)
>>>> @@ -658,6 +661,22 @@ static u64 pte_to_pagemap_entry(pte_t pte)
>>>>   	return pme;
>>>>   }
>>>>
>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>> +static u64 thp_pte_to_pagemap_entry(pte_t pte, int offset)
>>>> +{
>>>> +	u64 pme = 0;
>>>> +	if (pte_present(pte))
>>>
>>> When does pte_present() return 0?
>>
>> It does when the page pointed to by pte is swapped-out, under page migration,
>> or HWPOISONed. But currenly it can't happen on thp because thp will be
>> splitted before these operations are processed.
>> So this if-sentense is not necessary for now, but I think it's not a bad idea
>> to put it now to prepare for future implementation.
> 
> You certainly need to add a comment. otherwise you add *unnecessary* complexity
> and people is going to be puzzled.

OK, I care about that.
Thank you.
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