Re: [PATCH v8 06/23] mm/shmem: Handle uffd-wp special pte in page fault handler

David Hildenbrand
2022-05-11T16:31:17Z

> +/*
> + * This is actually a page-missing access, but with uffd-wp special pte
> + * installed. It means this pte was wr-protected before being unmapped.
> + */
> +static vm_fault_t pte_marker_handle_uffd_wp(struct vm_fault *vmf)
> +{
> + /*
> + * Just in case there're leftover special ptes even after the region
> + * got unregistered - we can simply clear them. We can also do that
> + * proactively when e.g. when we do UFFDIO_UNREGISTER upon some uffd-wp
> + * ranges, but it should be more efficient to be done lazily here.
> + */
> + if (unlikely(!userfaultfd_wp(vmf->vma) || vma_is_anonymous(vmf->vma)))
> + return pte_marker_clear(vmf);

What would happen if we do a unregister followed by a register? IMHO we
should start with a clean uffd-wp slate then. Your comment makes ma
assume that we could receive stale WP events, which would be wrong?

--
Thanks,

David / dhildenb