Date:	Fri, 14 Sep 2007 21:28:12 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Randy Dunlap <randy.dunlap@...cle.com>
cc:	lkml <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...e.de>
Subject: Re: crashme fault



On Wed, 12 Sep 2007, Randy Dunlap wrote:
>
> I run almost-daily kernel testing.  I haven't seen 'crashme' cause a
> kernel fault until today, and now I've seen it twice on 2.6.23-rc6-git2,
> x86_64.  After the first fault, I ran 'crashme' about 10 more times
> to get the second fault (usually for 10 minutes, one time for 30
> minutes).

Interesting. If this is reproducible for you, can you try to narrow down 
(with bit-bisect) roughly when it started.

> There is very little helpful info.  RIP is strange, e.g.: 000000000051b446
> The call stack is not printed.  No kernel symbols are printed,
> even though I have CONFIG_KALLSYMS{_ALL}=y.

It looks like it's a page fault as a result of a *user*space* access, and 
most likely your machine would continue happily, except you have 
"panic_on_oops" set, so when the oops happens, it shuts the system down.

Now, the reason I say it looks like a user space access is that you have

	RIP: 0033:[<0000000000510eea>]
	RSP: 002b:00007fffc9a8ec10

which are all user space segments. So the register contents clearly say 
"page fault in user space".

However, what makes the kernel oops (rather than just send a SIGSEGV) is 
that the page fault "error code" is zero (that's the number that is 
printed out just after the "Oops" string). For a normal user space access, 
you should have bit #2 set in the error code.

So the kernel thinks it's a kernel page fault, because the page fault 
error code says so. But everything else seems to indicate that it's really 
user mode.. It would be very interesting to hear when this started 
happening.

Even if you cannot bisect it down all the way, since you say that you do 
almost daily kernel testing, is this really new to 2.6.23-rc6-git2, and 
2.6.23-rc6-git1 was fine? 

Andi, anything comes to mind?

			Linus

---
> [ 7487.208128] Unable to handle kernel paging request at 00000000ff019b53 RIP:
> [ 7487.212752]  [<0000000000510eea>]
> [ 7487.218537] PGD 10c1a2067 PUD 0
> [ 7487.221811] Oops: 0000 [1] SMP
> [ 7487.224989] CPU 2
> [ 7487.227024] Modules linked in: loop
> [ 7487.230550] Pid: 19139, comm: crashme Not tainted 2.6.23-rc6-git2 #1
> [ 7487.236896] RIP: 0033:[<0000000000510eea>]  [<0000000000510eea>]
> [ 7487.242925] RSP: 002b:00007fffc9a8ec10  EFLAGS: 00010e83
> [ 7487.248234] RAX: 00000000ffff8c4a RBX: 00000000004014f1 RCX: 00002b20e11c8b37
> [ 7487.255361] RDX: 0000000000510ee0 RSI: 0000000000000000 RDI: 000000000000000a
> [ 7487.262489] RBP: 00007fffc9a8ec10 R08: 00007fffc9a8eb60 R09: 0000000000000000
> [ 7487.269616] R10: 0000000000000008 R11: 0000000000000612 R12: 0000000000000000
> [ 7487.276743] R13: 00007fffc9a8ee00 R14: 0000000000000000 R15: 0000000000000000
> [ 7487.283871] FS:  00002b20e13676d0(0000) GS:ffff81011fc75840(0000) knlGS:0000000000000000
> [ 7487.291952] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [ 7487.297693] CR2: 00000000ff019b53 CR3: 000000005be6b000 CR4: 00000000000006e0
> [ 7487.304821] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 7487.311949] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [ 7487.319076] Process crashme (pid: 19139, threadinfo ffff810106830000, task ffff810102cf5040)
> [ 7487.327511]
> [ 7487.329009] RIP  [<0000000000510eea>]
> [ 7487.332690]  RSP <00007fffc9a8ec10>
> [ 7487.336180] CR2: 00000000ff019b53
> [ 7487.339810] Kernel panic - not syncing: Fatal exception
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