Message-ID: <51FFA393.2080301@zytor.com>
Date: Mon, 05 Aug 2013 06:07:31 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Tang Chen <tangchen@cn.fujitsu.com>
CC: rjw@sisk.pl, lenb@kernel.org, tglx@linutronix.de, mingo@elte.hu,
        akpm@linux-foundation.org, tj@kernel.org, trenn@suse.de,
        yinghai@kernel.org, jiang.liu@huawei.com, wency@cn.fujitsu.com,
        laijs@cn.fujitsu.com, isimatu.yasuaki@jp.fujitsu.com,
        izumi.taku@jp.fujitsu.com, mgorman@suse.de, minchan@kernel.org,
        mina86@mina86.com, gong.chen@linux.intel.com,
        vasilis.liaskovitis@profitbricks.com, lwoodman@redhat.com,
        riel@redhat.com, jweiner@redhat.com, prarit@redhat.com,
        zhangyanfei@cn.fujitsu.com, yanghy@cn.fujitsu.com, x86@kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-acpi@vger.kernel.org
Subject: Re: [PATCH v2 00/18] Arrange hotpluggable memory as ZONE_MOVABLE.
References: <1375340800-19332-1-git-send-email-tangchen@cn.fujitsu.com>
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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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