linux-kernel - Re: questions about init_memory_mapping

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Message-ID: <4D6D4C9D.60403@zytor.com>
Date:	Tue, 01 Mar 2011 11:44:29 -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 03/01/2011 12:29 AM, Tejun Heo wrote:
> Hey,
> 
> (sorry about the earlier empty reply, fat finger on my phone)
> 
> On Mon, Feb 28, 2011 at 10:14:44AM -0800, H. Peter Anvin wrote:
>>> 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.)
> 
> Hmm... I don't follow.  Can you elaborate?  If PAE is not available
> for whatever reason, the physical memory is limited to 4GiB but I
> don't follow what that has to do with the above.
> 

Sorry, PSE, not PAE.

	-hpa
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