[MODERATED] Re: [PATCH v5 8/8] NX 8

On 10/10/19 18:50, Paolo Bonzini wrote:
> On 10/10/19 18:14, Paolo Bonzini wrote:
>> On 10/10/19 07:53, speck for Pawan Gupta wrote:
>>> We can help debug the crash. Can you please share the series,
>>> reproduction steps and the crash signature.
> 
> The bug is a race condition between kvm_mmu_zap_all and, well,
> everything else.  It is triggered when nx_huge_pages is clear/set while
> the recovery thread runs.

Gah, figured it out.  It's specific to v6.  I'll post the fixed patches
tomorrow, dinnertime now.

Paolo

> Paolo
> 
>> The reproduction steps for v5 are as follows:
>>
>> - grab the next branch of kvm-unit-tests.git[1] and build it
>>
>> - create a lot of hugepages, on my machine I use 40 GiB worth of them:
>>
>>   echo 20480 > /proc/sys/vm/nr_hugepages
>>
>> - load KVM with kvm.nx_huge_pages_recovery_period_secs=3
>>
>> - run the following script
>>
>>   while true; do
>>     echo N > /sys/module/kvm/parameters/nx_huge_pages; sleep 1
>>     echo Y > /sys/module/kvm/parameters/nx_huge_pages; sleep 5
>>   done
>>
>> - run the testcase with
>>
>>   MEM=40960  # in megabytes
>>   qemu-kvm -nodefaults -vnc none -serial stdio -kernel x86/hugetext.flat
>>     -m $MEM -mem-path /dev/hugepages
>>
>> You can also add a WARN_ON_ONCE(!sp->lpage_disallowed) to
>> kvm_recover_nx_lpages before the call to kvm_mmu_prepare_zap_page.  As
>> soon as it triggers, of course everything will go downhill.
>>
>> Paolo
>>
>> [1] git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git
>>
> 
>