linux-kernel - Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

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Date:	Mon, 3 Mar 2008 22:05:26 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Klaus S. Madsen" <ksm@...rnemadsen.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Suspend-devel list <suspend-devel@...ts.sourceforge.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

Hi!

> However I wasn't able to make the problem go away, by removing the
> _PAGE_PWT constants from __PAGE_KERNEL_NOCACHE and
> __PAGE_KERNEL_VSYSCALL_NOCACHE in include-asm/pgtable.h in the newest
> 2.6.25:
> 
> diff --git a/include/asm-x86/pgtable.h b/include/asm-x86/pgtable.h
> index 174b877..f81c968 100644
> --- a/include/asm-x86/pgtable.h
> +++ b/include/asm-x86/pgtable.h
> @@ -84,9 +84,9 @@ extern pteval_t __PAGE_KERNEL, __PAGE_KERNEL_EXEC;
>  #define __PAGE_KERNEL_RO		(__PAGE_KERNEL & ~_PAGE_RW)
>  #define __PAGE_KERNEL_RX		(__PAGE_KERNEL_EXEC & ~_PAGE_RW)
>  #define __PAGE_KERNEL_EXEC_NOCACHE	(__PAGE_KERNEL_EXEC | _PAGE_PCD | _PAGE_PWT)
> -#define __PAGE_KERNEL_NOCACHE		(__PAGE_KERNEL | _PAGE_PCD | _PAGE_PWT)
> +#define __PAGE_KERNEL_NOCACHE		(__PAGE_KERNEL | _PAGE_PCD)
>  #define __PAGE_KERNEL_VSYSCALL		(__PAGE_KERNEL_RX | _PAGE_USER)
> -#define __PAGE_KERNEL_VSYSCALL_NOCACHE	(__PAGE_KERNEL_VSYSCALL | _PAGE_PCD | _PAGE_PWT)
> +#define __PAGE_KERNEL_VSYSCALL_NOCACHE	(__PAGE_KERNEL_VSYSCALL | _PAGE_PCD)
>  #define __PAGE_KERNEL_LARGE		(__PAGE_KERNEL | _PAGE_PSE)
>  #define __PAGE_KERNEL_LARGE_EXEC	(__PAGE_KERNEL_EXEC | _PAGE_PSE)
>  
> So while I'm fairly confident in that I bisected correctly, the number
> of attempts I had to go through to get a reliable result, and the fact
> that I cannot make the problem go away by reverting the current code to
> something similar, counts quite a lot against me.
> 
> However I'm 100% confident that the problem appears between
> cf8fa920cb4271f17e0265c863d64bea1b31941a and
> 925596a017bbd045ff711b778256f459e50a119, which is something like 16
> commits. I have been at both points in the tree at least 2 times, and
> confirmed that cf8fa920cb4271f17e0265c863d64bea1b31941a worked for me,
> and 925596a017bbd045ff711b778256f459e50a119 didn't.
> 
> > while requiring PROT_EXEC is fine, breaking existing user-space apps 
> > over that is not fine. So are you absolutely sure that by reverting that 
> > PWT|PCD commit, s2ram again starts to work? That's utmost weird...
> I'm sure that it fixed the problem for me, yes, and I'm fairly confident
> that I ran make clean && make to compile the kernel during the entire
> bisection between the two commites mentioned above.
> 
> > perhaps there's some CPU bug that causes NX to _NOT_ work if only PCD is 
> > used (not PCD|PWT). Seems like a pretty unlikely scenario though.
> $ cat /proc/cpuinfo
> processor       : 0
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 15
> model name      : Intel(R) Core(TM)2 Duo CPU     T7500  @ 2.20GHz
> stepping        : 10
> 
> But I'm a bit puzzled by the fact that I'm aparently the only one how
> have encountered the problem? Maybe it's only a problem if one also uses
> PAE? (Thats just a wild guess to explain why I'm the only one seeing
> this).

I do not have NX capable CPU in machine using for suspend
testing. ... and you are using 32-bit userspace on 64-bit kernel,
right?

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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