On 27 November 2015 at 10:53, Tyler Baker <tyler.ba...@linaro.org> wrote:
> On 27 November 2015 at 09:08, Tyler Baker <tyler.ba...@linaro.org> wrote:
>> On 27 November 2015 at 00:54, Christian Borntraeger
>> <borntrae...@de.ibm.com> wrote:
>>> On 11/26/2015 09:47 PM, Christian Borntraeger wrote:
>>>> On 11/26/2015 05:17 PM, Tyler Baker wrote:
>>>>> Hi Christian,
>>>>> The kernelci.org bot recently has been reporting kvm guest boot
>>>>> failures[1] on various arm64 platforms in next-20151126. The bot
>>>>> bisected[2] the failures to the commit in -next titled "KVM: Create
>>>>> debugfs dir and stat files for each VM". I confirmed by reverting this
>>>>> commit on top of next-20151126 it resolves the boot issue.
>>>>> In this test case the host and guest are booted with the same kernel.
>>>>> The host is booted over nfs, installs qemu (qemu-system arm64 2.4.0),
>>>>> and launches a guest. The host is booting fine, but when the guest is
>>>>> launched it errors with "Failed to retrieve host CPU features!". I
>>>>> checked the host logs, and found an "Unable to handle kernel paging
>>>>> request" splat[3] which occurs when the guest is attempting to start.
>>>>> I scanned the patch in question but nothing obvious jumped out at me,
>>>>> any thoughts?
>>>> Not really.
>>>> Do you have processing running that do read the files in 
>>>> /sys/kernel/debug/kvm/* ?
>>>> If I read the arm oops message correctly it oopsed inside
>>>> __srcu_read_lock. there is actually nothing in there that can oops,
>>>> except the access to the preempt count. I am just guessing right now,
>>>> but maybe the preempt variable is no longer available (as the process
>>>> is gone). As long as a debugfs file is open, we hold a reference to
>>>> the kvm, which holds a reference to the mm, so the mm might be killed
>>>> after the process. But this is supposed to work, so maybe its something
>>>> different. An objdump of __srcu_read_lock might help.
>>> Hmm, the preempt thing is done in srcu_read_lock, but the crash is in
>>> __srcu_read_lock. This function gets the srcu struct from mmu_notifier.c,
>>> which must be present and is initialized during boot.
>>> int __srcu_read_lock(struct srcu_struct *sp)
>>> {
>>>         int idx;
>>>         idx = READ_ONCE(sp->completed) & 0x1;
>>>         __this_cpu_inc(sp->per_cpu_ref->c[idx]);
>>>         smp_mb(); /* B */  /* Avoid leaking the critical section. */
>>>         __this_cpu_inc(sp->per_cpu_ref->seq[idx]);
>>>         return idx;
>>> }
>>> Looking at the code I have no clue why the patch does make a difference.
>>> Can you try to get an objdump -S for__Srcu_read_lock?
> Some other interesting finding below...
> On the host, I do _not_ have any nodes under /sys/kernel/debug/kvm/
> Running strace on the qemu command I use to launch the guest yields
> the following.
> [pid  5963] 1448649724.405537 mmap(NULL, 65536, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6652a000
> [pid  5963] 1448649724.405586 read(13, "MemTotal:       16414616
> kB\nMemF"..., 1024) = 1024
> [pid  5963] 1448649724.405699 close(13) = 0
> [pid  5963] 1448649724.405755 munmap(0x7f6652a000, 65536) = 0
> [pid  5963] 1448649724.405947 brk(0x2552f000) = 0x2552f000
> [pid  5963] 1448649724.406148 openat(AT_FDCWD, "/dev/kvm",
> [pid  5963] 1448649724.406209 ioctl(13, KVM_CREATE_VM, 0) = -1 ENOMEM
> (Cannot allocate memory)

If I comment the call to kvm_create_vm_debugfs(kvm) the guest boots
fine. I put some printk's in the kvm_create_vm_debugfs() function and
it's returning -ENOMEM after it evaluates !kvm->debugfs_dentry. I was
chatting with some folks from the Linaro virtualization team and they
mentioned that ARM is a bit special as the same PID creates two vms in
quick succession, the first one is a scratch vm, and the other is the
'real' vm. With that bit of info, I suspect we may be trying to create
the debugfs directory twice, and the second time it's failing because
it already exists.


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