linux-kernel - Re: CONFIG_DEBUG_VIRTUAL instant reboot.

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Date:	Sat, 11 Jul 2015 10:13:22 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Jones <davej@...emonkey.org.uk>
cc:	x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: CONFIG_DEBUG_VIRTUAL instant reboot.

On Fri, 10 Jul 2015, Dave Jones wrote:
> I bought a Haswell Xeon E3-1225 v3 box, and found that
> my self-built kernel would instantly reboot the machine once
> grub had loaded it.
> 
> Through trial and error I found the following options matter..
> 
> if I _enable_ DEBUG_PAGEALLOC, it boots fine.
> disabling it again, instant death.
> 
> further narrowing down revealed that if I have DEBUG_PAGEALLOC off,
> I also _must_ also have CONFIG_DEBUG_VIRTUAL turned off.
> With both disabled, it also boots fine.
> 
> I tried earlyprintk, but this happens so soon I don't think
> we're even at the point of initializing that code.
> 
> Any ideas how I can further narrow this down ?

I assume that VIRTUAL_BUG_ON() triggers very early. Changing it to
WARN_ON() should keep the machine alive and give you a hint which of
those checks explodes.

Thanks,

	tglx
---
diff --git a/arch/x86/mm/physaddr.c b/arch/x86/mm/physaddr.c
index e666cbbb9261..e41c1227337f 100644
--- a/arch/x86/mm/physaddr.c
+++ b/arch/x86/mm/physaddr.c
@@ -76,7 +76,7 @@ unsigned long __phys_addr(unsigned long x)
 	/* max_low_pfn is set early, but not _that_ early */
 	if (max_low_pfn) {
 		VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);
-		BUG_ON(slow_virt_to_phys((void *)x) != phys_addr);
+		VIRTUAL_BUG_ON(slow_virt_to_phys((void *)x) != phys_addr);
 	}
 	return phys_addr;
 }
diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index 877ef226f90f..d7ff4c2ffd69 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -50,7 +50,7 @@ void dump_mm(const struct mm_struct *mm);
 #endif
 
 #ifdef CONFIG_DEBUG_VIRTUAL
-#define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
+#define VIRTUAL_BUG_ON(cond) WARN_ON(cond)
 #else
 #define VIRTUAL_BUG_ON(cond) do { } while (0)
 #endif
--
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