linux-kernel - Re: bisected boot regression post 2.6.25-rc3.. please revert

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Date:	Mon, 3 Mar 2008 22:13:24 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	hans.rosenfeld@....com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: bisected boot regression post 2.6.25-rc3.. please revert

>> hm. I suspect some gcc related difference related to the handling of 
>> this masking:
>>    pmd_val(x) & ~(PAGE_MASK | _PAGE_USER | _PAGE_PSE | _PAGE_NX)
>> versus:
>>    pmd_val(x) & (~PAGE_MASK & ~_PAGE_USER)
>> perhaps it will work if you change it to:
>>    pmd_val(x) & (~PAGE_MASK & ~_PAGE_USER & ~_PAGE_PSE & ~_PAGE_NX)
>> ?
>> in any case, the commit has to be reverted as it clearly isnt a NOP 
>> on your box as it was intended to be. (it should only have made a 
>> difference in a rare hugetlbfs case)
>
> interesting observation: if I turn the macros into inlines... the 
> difference goes away.

include/asm-x86/pgtable.h has

	#define _PAGE_BIT_PSE           7

	#define _PAGE_PSE       (_AC(1, L)<<_PAGE_BIT_PSE)

and

	#define _PAGE_BIT_NX           63

	#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
	#define _PAGE_NX        (_AC(1, ULL) << _PAGE_BIT_NX)

so (on 32-bit) ~_PAGE_PSE is ~0x80L is 0xffffff7f, which when cast to
64-bit is 0x00000000ffffff7f, so in

	(~PAGE_MASK & ~_PAGE_USER & ~_PAGE_PSE & ~_PAGE_NX)

all the high bits are lost, while the original

	~(PAGE_MASK | _PAGE_USER | _PAGE_PSE | _PAGE_NX)

works as intended, since the bit inversion is done on a 64-bit number.


Maybe all those pagetable bit definitions should use 64-bit (ULL or a 
cast),
as it is now some dangerous detail is hidden behind the macros.  Using 
inline
functions for simple constants seems like overkill to me, but would 
also work
of course.


Segher

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