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

Klaus S. Madsen
2008-03-03T20:52:38Z

On Mon, Mar 03, 2008 at 18:48:58 +0100, Ingo Molnar wrote:
>
> * Klaus S. Madsen <ksm@hjernemadsen.org> wrote:
>
> > The following patch solves the segfault, by changing the mmap flags of
> > the video memory area, to allow execution. The patch is against
> > libx86-0.99 available from http://www.codon.org.uk/~mjg59/libx86/
> >
> > --- libx86-0.99/x86-common.c 2006-09-08 00:44:27.000000000 +0200
> > +++ libx86-0.99.new/x86-common.c 2008-03-01 10:08:25.000000000 +0100
> > @@ -232,7 +232,7 @@
> > }
> >
> > m = mmap((void *)0xa0000, 0x100000 - 0xa0000,
> > - PROT_READ | PROT_WRITE,
> > + PROT_READ | PROT_WRITE | PROT_EXEC,
>
> are you sure you ID-ed the right commit that broke things?
I can't be sure. It was my third attempt, and there seems to be some
sort of Makefile trouble in that area, which causes the problem to
appear and disappear at random, unless I do a make clean && make. But
the triggering commit was found with make clean && make, and I made sure
that reverting the resulting commit did actually solve the problem...

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).

--
Kind regards
Klaus S. Madsen