linux-kernel - RE: [PATCH v2] X86-32: Allocate 256 bytes for pgd in PAE paging

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Message-ID: <3E5A0FA7E9CA944F9D5414FEC6C712205A6C1D8F@ORSMSX106.amr.corp.intel.com>
Date:	Thu, 18 Dec 2014 17:51:19 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Williamson, Glenn P" <glenn.p.williamson@...el.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH v2] X86-32: Allocate 256 bytes for pgd in PAE paging

> Sent: Thursday, December 18, 2014 9:26 AM
> To: Yu, Fenghua; Thomas Gleixner; H. Peter Anvin; Ingo Molnar; Williamson,
> Glenn P
> Cc: linux-kernel; x86
> Subject: Re: [PATCH v2] X86-32: Allocate 256 bytes for pgd in PAE paging
> 
> On 12/17/2014 01:47 PM, Fenghua Yu wrote:
> > From: Fenghua Yu <fenghua.yu@...el.com>
> >
> > X86 32-bit machine and kernel use PAE paging, which currently wastes
> > about 4K of memory per process on Linux where we have to reserve an
> > entire page to support a single 256-byte PGD structure.  It would be a
> > very good thing if we could eliminate that wastage.
> >
> 
> 4*8 = 32 bytes, where did 256 bytes come from?

You are right. It should be 32 bytes. I will change the wording in future patch. The real calculation in code is right though.

Thanks.

-Fenghua
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