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Date:      Sat, 29 Apr 2006 20:52:05 +0200
From:      Suleiman Souhlal <ssouhlal@FreeBSD.org>
To:        Nate Lawson <nate@root.org>
Cc:        src-committers@freebsd.org, cvs-all@freebsd.org, Peter Wemm <peter@wemm.org>, cvs-src@freebsd.org
Subject:   Re: cvs commit: src/sys/i386/i386 pmap.c
Message-ID:  <4453B5D5.4090206@FreeBSD.org>
In-Reply-To: <4453A429.5080406@root.org>
References:  <200604281905.k3SJ59MZ008806@repoman.freebsd.org> <20060429024850.GB997@turion.vk2pj.dyndns.org> <4452FB64.7030308@root.org> <200604291021.12379.peter@wemm.org> <4453A429.5080406@root.org>

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Nate Lawson wrote:
> Peter Wemm wrote:
> 
>> Stephan realized that the kernel already allocates one PTE per virtual 
>> page.  Although it normally holds physical addresses plus attributes, 
>> as long as we don't set PG_V, then there are 31 other bits that we 
>> could use for data storage.  We could put virtual addresses in there 
>> so long as we didn't set PG_V, and abuse that to have a singly linked 
>> freelist threaded through the PTEs.  It turned out even easier 
>> though.  As long as the virtual addresses are page aligned, we neatly 
>> avoid all the PG_* mode bits as well. 
> 
> 
> ... because you can shift by the page size, saving 12 bits.  But are 
> there any problems with 2 MB pages?

It should work fine with 2/4MB pages too. Just like PTEs, when a PDE 
(the 2/4MB entry) is marked as not present, all the bits but the zeroth 
one are available for use to the operating system.

-- Suleiman




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