linux-kernel - Re: [PATCH 2/2] x86, e820 disable ACPI Memory Hotplug if memory mapping is specified by user [v2]

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Message-ID: <CAHGf_=qWB81f8fdDdjaXXh1JoSDUsJmcEHwH+CEJ2E-5XWz6qA@mail.gmail.com>
Date:	Mon, 13 Jan 2014 15:31:04 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	Bodo Eggert <7eggert@....de>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linn Crosetto <linn@...com>, Pekka Enberg <penberg@...nel.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Toshi Kani <toshi.kani@...com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Vivek Goyal <vgoyal@...hat.com>, dyoung@...hat.com,
	linux-acpi@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] x86, e820 disable ACPI Memory Hotplug if memory
 mapping is specified by user [v2]

On Sun, Jan 12, 2014 at 6:46 PM, Prarit Bhargava <prarit@...hat.com> wrote:
>
>
> On 01/11/2014 11:35 AM, 7eggert@....de wrote:
>>
>>
>> On Fri, 10 Jan 2014, Prarit Bhargava wrote:
>>
>>> kdump uses memmap=exactmap and mem=X values to configure the memory
>>> mapping for the kdump kernel.  If memory is hotadded during the boot of
>>> the kdump kernel it is possible that the page tables for the new memory
>>> cause the kdump kernel to run out of memory.
>>>
>>> Since the user has specified a specific mapping ACPI Memory Hotplug should be
>>> disabled in this case.
>>
>> I'll ask just in case: Is it possible to want memory hotplug in spite of
>> using memmap=exactmap or mem=X?
>
> Good question -- I can't think of a case.  When a user specifies "memmap" or
> "mem" IMO they are asking for a very specific memory configuration.  Having
> extra memory added above what the user has specified seems to defeat the purpose
> of "memmap" and "mem".

May be yes, may be no.

They are often used for a wrokaround to avoid broken firmware issue.
If we have no way
to explicitly enable hotplug. We will lose a workaround.

Perhaps, there is no matter. Today, memory hotplug is only used on
high-end machine
and their firmware is carefully developped and don't have a serious
issue almostly. Though.
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