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: <1389664055.1792.269.camel@misato.fc.hp.com>
Date:	Mon, 13 Jan 2014 18:47:35 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Dave Young <dyoung@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Prarit Bhargava <prarit@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Bodo Eggert <7eggert@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.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>,
	Tang Chen <tangchen@...fujitsu.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Vivek Goyal <vgoyal@...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 Tue, 2014-01-14 at 09:52 +0800, Dave Young wrote:
> On 01/13/14 at 06:09pm, Toshi Kani wrote:
> > On Mon, 2014-01-13 at 16:53 -0800, H. Peter Anvin wrote:
> > > On 01/13/2014 04:33 PM, Toshi Kani wrote:
> > > > 
> > > > I do not think it makes sense.  You needed memmap=exactmap as a
> > > > workaround because the kernel did not boot with the firmware's memory
> > > > info.  So, it's broken, and you requested the kernel to ignore the
> > > > firmware info.
> > > > 
> > > > Why do you think memory hotplug needs to be supported under such
> > > > condition, which has to use the broken firmware info?
> > > > 
> > > 
> > > Even more than memory hotplug: what do we do with NUMA?  Since we have
> > > already told the kernel "the firmware is bogus" it would seem that any
> > > NUMA optimizations would be a bit ... cantankerous at best, no?
> > 
> > Agreed that NUMA info can be bogus in this case, but is probably not
> > critical.
> > 
> > In majority of the cases, memmap=exactmap is used for kdump and the
> > firmware info is sane.  So, I think we should keep NUMA enabled since it
> > could be useful when multiple CPUs are enabled for kdump.
> 
> In Fedora kdump, we by default add numa=off to 2nd kernel cmdline because
> enabling numa will use a lot more memory, at the same time we have only 128M
> reserved by default..

That quite makes sense as we only enable a single CPU today.

Thanks,
-Toshi

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