Re: [RFC PATCH v1 03/26] KVM: Add restricted support for mapping guestmem by the host

[David Hildenbrand <david@redhat.com>]

> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
> +{
> +	struct folio *folio;
> +
> +	folio = kvm_gmem_get_folio(file_inode(vmf->vma->vm_file), vmf->pgoff);
> +	if (!folio)
> +		return VM_FAULT_SIGBUS;
> +
> +	/*
> +	 * Check if the page is allowed to be faulted to the host, with the
> +	 * folio lock held to ensure that the check and incrementing the page
> +	 * count are protected by the same folio lock.
> +	 */
> +	if (!kvm_gmem_isfaultable(vmf)) {
> +		folio_unlock(folio);
> +		return VM_FAULT_SIGBUS;
> +	}
> +
> +	vmf->page = folio_file_page(folio, vmf->pgoff);

We won't currently get hugetlb (or even THP) here. It mimics what shmem 
would do.

finish_fault->set_pte_range() will call folio_add_file_rmap_ptes(), 
getting the rmap involved.

Do we have some tests in place that make sure that 
fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) will properly unmap 
the page again (IOW, that the rmap does indeed work?).

-- 
Cheers,

David / dhildenb