linux-kernel - Re: questions about init_memory_mapping

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Message-ID: <20110301082915.GC26074@htj.dyndns.org>
Date:	Tue, 1 Mar 2011 09:29:15 +0100
From:	Tejun Heo <tj@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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()

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.

Thanks.

-- 
tejun
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