linux-kernel - Re: pud_bad vs pud

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Date:	Thu, 5 Feb 2009 20:10:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	William Lee Irwin III <wli@...omorphy.com>,
	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:

> Ingo Molnar wrote:
>> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>>   
>>> I'm looking at unifying the 32 and 64-bit versions of pud_bad.
>>>
>>> 32-bits defines it as:
>>>
>>> static inline int pud_bad(pud_t pud)
>>> {
>>> 	return (pud_val(pud) & ~(PTE_PFN_MASK | _KERNPG_TABLE | _PAGE_USER)) != 0;
>>> }
>>>
>>> and 64 as:
>>>
>>> static inline int pud_bad(pud_t pud)
>>> {
>>> 	return (pud_val(pud) & ~(PTE_PFN_MASK | _PAGE_USER)) != _KERNPG_TABLE;
>>> }
>>>
>>>
>>> I'm inclined to go with the 64-bit version, but I'm wondering if 
>>> there's something subtle I'm missing here.
>>>     
>>
>> Why go with the 64-bit version? The 32-bit check looks more compact and 
>> should result in smaller code.
>>   
>
> Well, its stricter.  But I don't really understand what condition its  
> actually testing for.

Well it tests: "beyond the bits covered by PTE_PFN|_PAGE_USER, the rest 
must only be _KERNPG_TABLE".

The _KERNPG_TABLE bits are disjunct from PTE_PFN|_PAGE_USER bits, so this 
makes sense.

But the 32-bit check does the exact same thing but via a single binary 
operation: it checks whether any bits outside of those bits are zero - just 
via a simpler test that compiles to more compact code.

So i'd go with the 32-bit version. (unless there are some sign-extension 
complications i'm missing - but i think we got rid of those already.)

	Ingo
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