linux-kernel - Re: [PATCH] x86/mm: fix regression with huge pages on PAE

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Message-ID: <20151112080059.GA6835@gmail.com>
Date:	Thu, 12 Nov 2015 09:00:59 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Borislav Petkov <bp@...en8.de>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, jgross@...e.com,
	konrad.wilk@...cle.com, elliott@....com,
	boris.ostrovsky@...cle.com, Toshi Kani <toshi.kani@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/mm: fix regression with huge pages on PAE


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> On Thu, Nov 12, 2015 at 08:48:54AM +0100, Ingo Molnar wrote:
> > 
> > * Borislav Petkov <bp@...en8.de> wrote:
> > 
> > > --- a/arch/x86/include/asm/pgtable_types.h
> > > +++ b/arch/x86/include/asm/pgtable_types.h
> > > @@ -279,17 +279,14 @@ static inline pmdval_t native_pmd_val(pmd_t pmd)
> > >  static inline pudval_t pud_pfn_mask(pud_t pud)
> > >  {
> > >  	if (native_pud_val(pud) & _PAGE_PSE)
> > > -		return PUD_PAGE_MASK & PHYSICAL_PAGE_MASK;
> > > +		return ~((1ULL << PUD_SHIFT) - 1) & PHYSICAL_PAGE_MASK;
> > >  	else
> > >  		return PTE_PFN_MASK;
> > >  }
> > 
> > >  static inline pmdval_t pmd_pfn_mask(pmd_t pmd)
> > >  {
> > >  	if (native_pmd_val(pmd) & _PAGE_PSE)
> > > -		return PMD_PAGE_MASK & PHYSICAL_PAGE_MASK;
> > > +		return ~((1ULL << PMD_SHIFT) - 1) & PHYSICAL_PAGE_MASK;
> > >  	else
> > >  		return PTE_PFN_MASK;
> > >  }
> > 
> > So instead of uglifying the code, why not fix the real bug: change the 
> > PMD_PAGE_MASK/PUD_PAGE_MASK definitions to be 64-bit everywhere?
> 
> *PAGE_MASK are usually applied to virtual addresses. I don't think it
> should anything but 'unsigned long'. This is odd use case really.

So we already have PHYSICAL_PAGE_MASK, why not introduce PHYSICAL_PMD_MASK et al, 
instead of uglifying the code?

But, what problems do you expect with having a wider mask than its primary usage? 
If it's used for 32-bit values it will be truncated down safely. (But I have not 
tested it, so I might be missing some complication.)

Thanks,

	Ingo
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