On 27 November 2015 at 09:08, Tyler Baker <tyler.baker@xxxxxxxxxx> wrote:
> On 27 November 2015 at 00:54, Christian Borntraeger
> <borntraeger@xxxxxxxxxx> 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",
O_RDWR|O_CLOEXEC) = 13
[pid 5963] 1448649724.406209 ioctl(13, KVM_CREATE_VM, 0) = -1 ENOMEM
(Cannot allocate memory)
[pid 5963] 1448649724.406382 close(13) = 0
[pid 5963] 1448649724.406435 write(2, "Failed to retrieve host CPU
feat"..., 38Failed to retrieve host CPU features!
) = 38
Tyler
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
