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

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Date:	Mon, 05 Aug 2013 06:07:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Tang Chen <tangchen@...fujitsu.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,
	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 v2 00/18] Arrange hotpluggable memory as ZONE_MOVABLE.

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

	-hpa


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