linux-kernel - Re: [PATCH v5 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up

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Date:	Thu, 26 Sep 2013 15:52:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Zhang Yanfei <zhangyanfei.yes@...il.com>
Cc:	"Rafael J . Wysocki" <rjw@...k.pl>, lenb@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>, mingo@...e.hu,
	"H. Peter Anvin" <hpa@...or.com>, Tejun Heo <tj@...nel.org>,
	Toshi Kani <toshi.kani@...com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Thomas Renninger <trenn@...e.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Jiang Liu <jiang.liu@...wei.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	isimatu.yasuaki@...fujitsu.com, izumi.taku@...fujitsu.com,
	Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
	mina86@...a86.com, gong.chen@...ux.intel.com,
	vasilis.liaskovitis@...fitbricks.com, lwoodman@...hat.com,
	Rik van Riel <riel@...hat.com>, jweiner@...hat.com,
	prarit@...hat.com, "x86@...nel.org" <x86@...nel.org>,
	linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, linux-acpi@...r.kernel.org,
	imtangchen@...il.com, Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH v5 4/6] x86/mem-hotplug: Support initialize page tables
 in bottom-up

On Wed, 25 Sep 2013 02:30:51 +0800 Zhang Yanfei <zhangyanfei.yes@...il.com> wrote:

> From: Tang Chen <tangchen@...fujitsu.com>
> 
> The Linux kernel cannot migrate pages used by the kernel. As a
> result, kernel pages cannot be hot-removed. So we cannot allocate
> hotpluggable memory for the kernel.
> 
> In a memory hotplug system, any numa node the kernel resides in
> should be unhotpluggable. And for a modern server, each node could
> have at least 16GB memory. So memory around the kernel image is
> highly likely unhotpluggable.
> 
> ACPI SRAT (System Resource Affinity Table) contains the memory
> hotplug info. But before SRAT is parsed, memblock has already
> started to allocate memory for the kernel. So we need to prevent
> memblock from doing this.
> 
> So direct memory mapping page tables setup is the case. init_mem_mapping()
> is called before SRAT is parsed. To prevent page tables being allocated
> within hotpluggable memory, we will use bottom-up direction to allocate
> page tables from the end of kernel image to the higher memory.
> 
> ...
>
> +		kernel_end = __pa_symbol(_end);

__pa_symbol() is implemented only on mips and x86.

I stole the mips implementation like this:

--- a/mm/memblock.c~a
+++ a/mm/memblock.c
@@ -187,8 +187,11 @@ phys_addr_t __init_memblock memblock_fin
 	/* avoid allocating the first page */
 	start = max_t(phys_addr_t, start, PAGE_SIZE);
 	end = max(start, end);
+#ifdef CONFIG_X86
 	kernel_end = __pa_symbol(_end);
-
+#else
+	kernel_end = __pa(RELOC_HIDE((unsigned long)(_end), 0));
+#endif
 	/*
 	 * try bottom-up allocation only when bottom-up mode
 	 * is set and @end is above the kernel image.

just so I can get a -mm release out the door.
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