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

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Date:	Mon, 12 Aug 2013 12:22:47 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Tang Chen <imtangchen@...il.com>
Cc:	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.

Hello, Tang.

On Tue, Aug 13, 2013 at 12:19:02AM +0800, Tang Chen wrote:
> The kernel can export info to users. The point is what kind of info.
> Exporting phys addr is meaningless, of course. Now in /sys, we only
> have memory_block and node. memory_block is only 128M on x86, and
> hotplug a memory_block means nothing. So actually we only have node.
> 
> So users want to hotplug a node is reasonable, I think. In the
> beginning, we set the hotplug unit to a node. That is also why we
> did the movable node.
> 
> In summary, node hotplug is much meaningful and usable for users.
> So it is the best that we can arrange a whole node to be movable
> node, not opportunistic.

Still not following.  Yeah, sure, you can tell the userland that node
X is hotpluggable or not hotpluggable after boot is complete.  Why is
that relevant?

> I'm just thinking of a more extreme case. For example, if a machine
> has only one node hotpluggable, and the kernel resides in that node.
> Then the system has no hotpluggable node.

Yeah, sure, then there's no way that node can be hotpluggable and the
right thing to do is booting up the machine and informing the userland
that memory is not hotpluggable.

> If we can prevent the kernel from using hotpluggable memory, in such
> a machine, users can still do memory hotplug.
> 
> I wanted to do it as generic as possible. But yes, finding out the
> nodes the kernel resides in and make it unhotpluggable can work.

Short of being able to remap memory under the kernel, I don't think
this can be very generic and as a compromise trying to keep as many
hotpluggable nodes as possible doesn't sound too bad.

Thanks.

-- 
tejun
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