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: <52D4793E.8070102@redhat.com>
Date:	Mon, 13 Jan 2014 18:39:42 -0500
From:	Prarit Bhargava <prarit@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.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 01/13/2014 03:31 PM, KOSAKI Motohiro wrote:
> 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.

Oof -- sorry Kosaki :(  I didn't see this until just now (and your subsequent
ACK on the updated patch).

I just remembered that we did have a processor vendor's whitebox that would not
boot unless we specified a specific memmap and we did specify memmap=exactmap to
boot the system correctly and the system had hotplug memory.

So it means that I should not key off of "memmap=exactmap".

I will self-NAK the updated patch and submit a new one.

P.


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