linux-kernel - Re: pud_bad vs pud

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Message-ID: <20090205205735.GA21500@elte.hu>
Date:	Thu, 5 Feb 2009 21:57:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Hugh Dickins <hugh@...itas.com>,
	William Lee Irwin III <wli@...ementarian.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: pud_bad vs pud_bad


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Hugh Dickins wrote:
>> However... I forget how the folding works out.  The pgd in the 32-bit
>> PAE case used to have just the pfn and the present bit set in that
>> little array of four entries: if pud_bad() ends up getting applied
>> to that, I guess it will blow up.
>>   
>
> Ah, that's a good point.
>
>> If so, my preferred answer would actually be to make those 4 entries
>> look more like real ptes; but you may think I'm being a bit silly.
>
> Hardware doesn't allow it.  It will explode (well, trap) if you set  
> anything other than P in the top level.

Yeah. I was the first Linux hacker in history to put a x86 CPU into PAE mode 
under Linux 10+ years ago, and i can attest to the 'explodes way too easily' 
aspect quite emphatically ;-) Took me 3-4 days to bootstrap it.

> By the by, what are the chances we'll be able to deprecate non-PAE 32-bit?

For the next 10 years: pretty much zero.

	Ingo
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