Revert "vfio/type1: Unpin zero pages"

This reverts commit 873aefb.

This was a heinous workaround and it turns out it's been fixed in mm
twice since it was introduced.  Most recently, commit c8070b7
("mm: Don't pin ZERO_PAGE in pin_user_pages()") would have prevented
running up the zeropage refcount, but even before that commit
84209e8 ("mm/gup: reliable R/O long-term pinning in COW mappings")
avoids the vfio use case from pinning the zeropage at all, instead
replacing it with exclusive anonymous pages.

Remove this now useless overhead.

Suggested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240229223544.257207-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: awilliam

drivers/vfio/vfio_iommu_type1.c

@@ -567,18 +567,6 @@ static int vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
 	ret = pin_user_pages_remote(mm, vaddr, npages, flags | FOLL_LONGTERM,
 				    pages, NULL);
 	if (ret > 0) {
-		int i;
-
-		/*
-		 * The zero page is always resident, we don't need to pin it
-		 * and it falls into our invalid/reserved test so we don't
-		 * unpin in put_pfn().  Unpin all zero pages in the batch here.
-		 */
-		for (i = 0 ; i < ret; i++) {
-			if (unlikely(is_zero_pfn(page_to_pfn(pages[i]))))
-				unpin_user_page(pages[i]);
-		}
-
 		*pfn = page_to_pfn(pages[0]);
 		goto done;
 	}