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Date: Mon, 29 Nov 2010 17:59:25 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
To: Mel Gorman <mel@csn.ul.ie>
Cc: linux-mm@kvack.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Chris Mason <chris.mason@oracle.com>, Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 18 of 66] add pmd mangling functions to x86
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On Mon, Nov 29, 2010 at 10:23:11AM +0000, Mel Gorman wrote:
> > > > @@ -353,7 +353,7 @@ static inline unsigned long pmd_page_vad
> > > >   * Currently stuck as a macro due to indirect forward reference to
> > > >   * linux/mmzone.h's __section_mem_map_addr() definition:
> > > >   */
> > > > -#define pmd_page(pmd)	pfn_to_page(pmd_val(pmd) >> PAGE_SHIFT)
> > > > +#define pmd_page(pmd)	pfn_to_page((pmd_val(pmd) & PTE_PFN_MASK) >> PAGE_SHIFT)
> > > >  
> > > 
> > > Why is it now necessary to use PTE_PFN_MASK?
> > 
> > Just for the NX bit, that couldn't be set before the pmd could be
> > marked PSE.
> > 
> 
> Sorry, I still am missing something. PTE_PFN_MASK is this
> 
> #define PTE_PFN_MASK            ((pteval_t)PHYSICAL_PAGE_MASK)
> #define PHYSICAL_PAGE_MASK      (((signed long)PAGE_MASK) & __PHYSICAL_MASK)
> 
> I'm not seeing how PTE_PFN_MASK affects the NX bit (bit 63).

It simply clears it by doing & 0000... otherwise bit 51 would remain
erroneously set on the pfn passed to pfn_to_page.

Clearing bit 63 wasn't needed before because bit 63 couldn't be set on
a not huge pmd.