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

[David Hildenbrand]

> +/*
> + * 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