linux-kernel - Re: [PATCH] x86/mm/init: respect memblock reserved regions when destroying mappings

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Date:	Mon, 7 Feb 2011 16:02:55 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH] x86/mm/init: respect memblock reserved regions when
 destroying mappings

On Sat, 5 Feb 2011, Jeremy Fitzhardinge wrote:
> >  		pud = pud_offset(pgd_offset_k(_brk_end), _brk_end);
> >  		pmd = pmd_offset(pud, _brk_end - 1);
> > -		while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1))
> > -			pmd_clear(pmd);
> > +		addr = (_brk_end + PMD_SIZE - 1) & PMD_MASK;
> 
> I guess its OK if this is >_end, because the pmd offset will be greater
> than _end's.  But is there an edge condition if the pmd_offset goes off
> the end of the pud, and pud page itself happens to be at the end of the
> address space and it wraps?
> 
> > +		while (++pmd <= pmd_offset(pud, (unsigned long)_end - 1)) {
> Could _end be in a different pud from _brk_end?  Could this walk off the
> pud page?
> 
> Or is it moot, and there's a guarantee that the whole space is mapped
> out of the same pud page?  I guess the original code has the same
> concern, so this patch leaves the status quo unchanged.
> 

Indeed. I had this doubt myself, but I thought there must have been a
smart reason why _brk_end and _end must be on the same pud, so I left it
unchanged.
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