linux-kernel - Re: [RFC][PATCH 2/3] pagemap: export KPF

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Message-ID: <4EEF9F3E.9000107@linux.vnet.ibm.com>
Date:	Mon, 19 Dec 2011 12:31:58 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/3] pagemap: export KPF_THP

On 12/19/2011 11:24 AM, KOSAKI Motohiro wrote:
> (12/19/11 1:38 PM), Naoya Horiguchi wrote:
>> This flag shows that a given pages is a subpage of transparent hugepage.
>> It does not care about whether it is a head page or a tail page, because
>> it's clear from pfn of the target page which you should know when you read
>> /proc/kpageflags.
>>
>> Signed-off-by: Naoya Horiguchi<n-horiguchi@...jp.nec.com>
> 
> NAK.
> 
> The detail of transparent hugepage are hidden by design. We hope it
> keep 'transparent'.
> Until any explain why we should expose KPF_THP, we don't agree it.

Transparent shouldn't mean "undebuggable", though. :)

Let's say you profiled a application and the data shows you're missing
the TLB a bunch, but you're also using THP.  This might give you a shot
at figuring out which parts of your application are *TRULY* THP-backed
instead of just the areas you *think* are backed.

I'm not sure there's another way to figure it out at the moment.

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