linux-kernel - Re: questions about init_memory_mapping

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Message-ID: <4D6BE614.3010007@zytor.com>
Date:	Mon, 28 Feb 2011 10:14:44 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Yinghai Lu <yinghai@...nel.org>, x86@...nel.org,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: questions about init_memory_mapping_high()

On 02/23/2011 09:19 AM, Tejun Heo wrote:
> Hello, guys.
> 
> I've been looking at init_memory_mapping_high() added by commit
> 1411e0ec31 (x86-64, numa: Put pgtable to local node memory) and I got
> curious about several things.
> 
> 1. The only rationale given in the commit description is that a
>    RED-PEN is killed, which was the following.
> 
> 	/*
> 	 * RED-PEN putting page tables only on node 0 could
> 	 * cause a hotspot and fill up ZONE_DMA. The page tables
> 	 * need roughly 0.5KB per GB.
> 	 */
> 
>    This already wasn't true with top-down memblock allocation.
> 
>    The 0.5KB per GiB comment is for 32bit w/ 3 level mapping.  On
>    64bit, it's ~4KiB per GiB when using 2MiB mappings and, well, very
>    small per GiB if 1GiB mapping is used.  Even with 2MiB mapping,
>    1TiB mapping would only be 4MiB.  Under ZONE_DMA, this could be
>    problematic but with top-down this can't be a problem in any
>    realistic way in foreseeable future.
> 

It's true on 64 bits too when PAE is not available (e.g. with Xen.)

	-hpa
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