linux-kernel - Re: [PATCH part5 0/7] Arrange hotpluggable memory as ZONE

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Date:	Wed, 14 Aug 2013 15:40:31 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Tang Chen <imtangchen@...il.com>,
	Tang Chen <tangchen@...fujitsu.com>, robert.moore@...el.com,
	lv.zheng@...el.com, rjw@...k.pl, lenb@...nel.org,
	tglx@...utronix.de, mingo@...e.hu, hpa@...or.com,
	akpm@...ux-foundation.org, trenn@...e.de, yinghai@...nel.org,
	jiang.liu@...wei.com, wency@...fujitsu.com, laijs@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, izumi.taku@...fujitsu.com,
	mgorman@...e.de, minchan@...nel.org, mina86@...a86.com,
	gong.chen@...ux.intel.com, vasilis.liaskovitis@...fitbricks.com,
	lwoodman@...hat.com, riel@...hat.com, jweiner@...hat.com,
	prarit@...hat.com, zhangyanfei@...fujitsu.com,
	yanghy@...fujitsu.com, x86@...nel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH part5 0/7] Arrange hotpluggable memory as ZONE_MOVABLE.

(8/14/13 2:23 PM), Tejun Heo wrote:
> Hello,
>
> On Wed, Aug 14, 2013 at 02:15:44PM -0400, KOSAKI Motohiro wrote:
>> I don't follow this. We need to think why memory hotplug is necessary.
>> Because system reboot is unacceptable on several critical services. Then,
>> if someone set wrong boot option, systems SHOULD fail to boot. At that time,
>> admin have a chance to fix their mistake. In the other hand, after running
>> production service, they have no chance to fix the mistake. In general, default
>> boot option should have a fallback and non-default option should not have a
>> fallback. That's a fundamental rule.
>
> The fundamental rule is that the system has to boot.

I don't agree it. Please look at other kernel options. A lot of these don't
follow you. These behave as direction, not advise.

I mean the fallback should be implemented at turning on default the feature.


>  Your argument is
> pointless as the kernel has no control over where its own image is
> placed w.r.t. hotpluggable nodes.  So, are we gonna fail boot if
> kernel image intersects hotpluggable node and the option is specified
> even if memory hotplug can be used on other nodes?  That doesn't make
> any sense.

I don't read whole discussion and I don't quite understand why no kernel
place controlling is relevant. Every unpluggable node is suitable for
kernel. If you mean current kernel placement logic don't care plugging,
that's a bug.

If we aim to hot remove, we have to have either kernel relocation or
hotplug awre kernel placement at boot time.

> Failing to boot is *way* worse reporting mechanism than almost
> everything else.  If the sysadmin is willing to risk machines failing
> to come up, she would definitely be willing to check whether which
> memory areas are actually hotpluggable too, right?

No. see above. Your opinion is not pragmatic useful.


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