Hi,
Perhaps try the entire series?
Latest version is at: https://git.kraxel.org/cgit/linux/log/?h=drm-virtio-no-wc
Applied entire series on top of 5.5.6, but still the same problem.
Can you double-check? Cherry-picking the shmem + virtio patches ...
694c824491f1 drm/shmem: add support for per object caching flags. 3f4d10f853fb drm/virtio: fix mmap page attributes
... into 5.6-rc3 fixes the corruption for me.
cheers, Gerd