linux-kernel - Re: [PATCH v2 00/18] Arrange hotpluggable memory as ZONE

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Date:	Mon, 05 Aug 2013 21:38:25 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	rjw@...k.pl, lenb@...nel.org, tglx@...utronix.de, mingo@...e.hu,
	akpm@...ux-foundation.org, tj@...nel.org, trenn@...e.de,
	yinghai@...nel.org, jiang.liu@...wei.com, wency@...fujitsu.com,
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	izumi.taku@...fujitsu.com, mgorman@...e.de, minchan@...nel.org,
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	vasilis.liaskovitis@...fitbricks.com, lwoodman@...hat.com,
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	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 v2 00/18] Arrange hotpluggable memory as ZONE_MOVABLE.


Hi hpa,

I'm sorry but I don't quite following it.

On 08/05/2013 09:07 PM, H. Peter Anvin wrote:
> On 08/01/2013 12:06 AM, Tang Chen wrote:
>> This patch-set aims to solve some problems at system boot time
>> to enhance memory hotplug functionality.
>>
>> [Background]
>>
>> The Linux kernel cannot migrate pages used by the kernel because
>> of the kernel direct mapping. Since va = pa + PAGE_OFFSET, if the
>> physical address is changed, we cannot simply update the kernel
>> pagetable. On the contrary, we have to update all the pointers
>> pointing to the virtual address, which is very difficult to do.
>>
>
> It does beg the question if that "since" statement should be changed ...

Do you mean we are going to do kernel page migration in the future ?

> we already have it handled differently on Xen PV, but that is kind of
> "special".  There are a whole bunch of other issues with moving kernel
> memory around: you have to worry what might have a physical address
> cached somewhere and what might be in active use and so on...

The current solution is to hotplug ZONE_MOVABLE, which the kernel won't
use. So do you mean if I want to do kernel page migration (which I'm not
doing), I need to worry about what you said above ?

>I am not
> really suggesting it as anything but food for thought at this time.

Sorry for my poor English, and I really cannot understand this one.

Thanks.
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