From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: 3.9 / 3.10: Reliable host crashes
Date: Wed, 10 Jul 2013 19:02:19 +0300
Message-ID: <20130710160218.GA24941@redhat.com>
References: <krjqc7$a8c$1@ger.gmane.org>
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Cc: kvm@vger.kernel.org
To: Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>
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On Wed, Jul 10, 2013 at 04:16:46PM +0200, Bernd Schubert wrote:
> Hi all,
> 
> I found a way to reliably crash my host system:
> 
> 1) Boot guest VM with init=/bin/bash
> 
> 2) In guest VM: echo b >/proc/sysrq-trigger
> 
> 3) Try to reboot the guest -> crashes the host during kernel initialization
> 
What 3 means? 2 already reboots it.

> 
> When I checked the logs I first thought it would be a KSM issue:
> 
> >Jul 10 15:33:37 fsdevel7 kernel: [  507.995602] br0: port 3(tap2) entered disabled state
> >Jul 10 15:33:37 fsdevel7 kernel: [  508.043454] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
> >Jul 10 15:33:37 fsdevel7 kernel: [  508.043487] IP: [<ffffffff811825b9>] get_ksm_page+0x39/0x130
> 
> and
> 
> 
> >Jul 10 15:34:49 fsdevel7 kernel: [  580.274709] RIP: 0010:[<ffffffff8116d694>]  [<ffffffff8116d694>] anon_vma_clone+0x94/0x1a0
> >Jul 10 15:34:49 fsdevel7 kernel: [  580.274710] RSP: 0018:ffff880406495cf0  EFLAGS: 00010286
> 
> >Jul 10 15:34:49 fsdevel7 kernel: [  580.274721] Call Trace:
> >Jul 10 15:34:49 fsdevel7 kernel: [  580.274724]  [<ffffffff8116d868>] anon_vma_fork+0x38/0x130
> >Jul 10 15:34:49 fsdevel7 kernel: [  580.274728]  [<ffffffff8105770f>] dup_mmap+0x1bf/0x420
> 
> 
> But then I simply disabled KSM in rc.local
> (echo 0 > /sys/kernel/mm/ksm/run) before starting the VMs. Now the
> system still crashes, but now without any logs message.
> At least sysrq-b still works, but I don' see any
> "SysRq : Emergency Sync" messages in log files. So log messages are
> not written properly anymore. Unfortunately I don't have a serial
> console on my desktop system.
> 
> I don't have the time now, but the next step is probably to get a
> crash-dump and to see if that has something useful. And I'm also
> going to run the VM from a tty without X, maybe it prints something
> to the console.
> Anything else you might want? Or already any idea?
> Crashes do not happen with 3.8-ubuntu, but very reliably with 3.9.9
> or 3.10.1 (host).
> 
What is your host cpu? cat /proc/cpuinfo.

> Here's my kvm command line:
> 
> >qemu-system-x86_64                                                      \
> >        -m 8192                                                         \
> >        -machine type=pc,accel=kvm,kernel_irqchip=on                    \
> >        -netdev type=tap,script=${kvm_ifup},downscript=${kvm_ifdown},ifname=$iface,vhost=on,id=guest0   \
> >        -device virtio-net-pci,netdev=guest0,mac=52:54:00:12:34:11      \
> >        -boot c                                                         \
> >        -drive file=${FILE},if=${DISKIF},cache=unsafe                   \
> >        -drive file=${META},if=${DISKIF},cache=writeback                \
> >        -drive file=${STORAGE},if=${DISKIF},cache=writeback             \
> >        -enable-kvm                                                     \
> >        -vga vmware                                                     \
> >        -cpu host                                                       \
> >        -smp 4                                                          \
> >        "$@"
> 
> 
> 
> Thanks,
> Bernd
> 
> --
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--
			Gleb.