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Date: Mon, 19 Dec 2011 12:31:58 -0800
From: Dave Hansen <dave@linux.vnet.ibm.com>
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To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
CC: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, linux-mm@kvack.org,
        Andi Kleen <andi@firstfloor.org>,
        Wu Fengguang <fengguang.wu@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 2/3] pagemap: export KPF_THP
References: <1324319919-31720-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1324319919-31720-3-git-send-email-n-horiguchi@ah.jp.nec.com> <4EEF8F85.9010408@gmail.com>
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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@ah.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.