Re: [PATCH v1 0/2] mm: don't install PMD mappings when THPs are disabled by the hw/process/vma

[David Hildenbrand <david@redhat.com>]

On 11.10.24 13:39, Thomas Huth wrote:
> On 11/10/2024 12.24, David Hildenbrand wrote:
>> During testing, it was found that we can get PMD mappings in processes
>> where THP (and more precisely, PMD mappings) are supposed to be disabled.
>> While it works as expected for anon+shmem, the pagecache is the problematic
>> bit.
>>
>> For s390 KVM this currently means that a VM backed by a file located on
>> filesystem with large folio support can crash when KVM tries accessing
>> the problematic page, because the readahead logic might decide to use
>> a PMD-sized THP and faulting it into the page tables will install a
>> PMD mapping, something that s390 KVM cannot tolerate.
>>
>> This might also be a problem with HW that does not support PMD mappings,
>> but I did not try reproducing it.
>>
>> Fix it by respecting the ways to disable THPs when deciding whether we
>> can install a PMD mapping. khugepaged should already be taking care of
>> not collapsing if THPs are effectively disabled for the hw/process/vma.
>>
>> An earlier patch was tested by Thomas Huth, this one still needs to
>> be retested; sending it out already.
> 
> I just finished testing your new version of these patches here, and I can
> confirm that they are fixing the problem that I was facing, so:
> 
> Tested-by: Thomas Huth <thuth@redhat.com>
> 
> FWIW, the problem can be reproduced by running a KVM guest on a s390x host
> like this:
> 
> qemu-system-s390x -accel kvm -nographic -m 4G -d guest_errors \
>     -M s390-ccw-virtio,memory-backend=mem-machine_mem \
>     -object
> memory-backend-file,size=4294967296,prealloc=true,mem-path=$HOME/myfile,share=true,id=mem-machine_mem
> 
> Without the fix, the guest crashes immediatly before being able to execute
> the first instruction. With the fix applied, you can still see the first
> messages of the guest firmware, indicating that the guest started successfully.
> 
> Thank you very much for the fix, David!

Thanks for the quick test, Thomas!

-- 
Cheers,

David / dhildenb