linux-kernel - Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE

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Message-ID: <51479A36.3050600@zytor.com>
Date:	Mon, 18 Mar 2013 15:50:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Lin Feng <linfeng@...fujitsu.com>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, penberg@...nel.org,
	jacob.shin@....com
Subject: Re: [PATCH] x86: mm: accurate the comments for STEP_SIZE_SHIFT macro

On 03/18/2013 02:19 PM, Yinghai Lu wrote:
> On Mon, Mar 18, 2013 at 12:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 
>> Instead, try to explain why 5 is the correct value in the current code
>> and how it is (or should be!) derived.
> 
> initial mapped size is PMD_SIZE, aka 2M.
> if we use step_size to be PUD_SIZE aka 1G, as most worse case
> that 1G is cross the 1G boundary, and PG_LEVEL_2M is not set,
> we will need 1+1+512 pages (aka 2M + 8k) to map 1G range with PTE.
> So i picked (30-21)/2 to get 5.
> 
> Please check attached patch.
> 
> Thanks
> 
> Yinghai
> 

This still seems very opaque.  I need to look at it and see if it makes
more sense in context.

	-hpa

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