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

[David Hildenbrand]

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@xxxxxxxxxx>
>
> 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