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

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Message-ID: <CAE9FiQU2iqx=9LEx_u6J5O_kQ-5Lo6DTgSgnk71k0p6WWUa7Hg@mail.gmail.com>
Date:	Mon, 18 Mar 2013 14:19:57 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 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

Download attachment "add_comment_for_step_size.patch" of type "application/octet-stream" (1551 bytes)