Function vm_object_split

freebsd/vm/vm_object.c:1497–1646

* vm_object_split: * * Split the pages in a map entry into a new object. This affords * easier removal of unused pages, and keeps object inheritance from * being a negative impact on memory usage. */

 * being a negative impact on memory usage.
 */
void
vm_object_split(vm_map_entry_t entry)
{
	vm_page_t m, m_busy, m_next;
	vm_object_t orig_object, new_object, backing_object;
	vm_pindex_t idx, offidxstart;
	vm_size_t size;

	orig_object = entry->object.vm_object;
	KASSERT((orig_object->flags & OBJ_ONEMAPPING) != 0,
	    ("vm_object_split:  Splitting object with multiple mappings."));
	if ((orig_object->flags & OBJ_ANON) == 0)
		return;
	if (orig_object->ref_count <= 1)
		return;
	VM_OBJECT_WUNLOCK(orig_object);

	offidxstart = OFF_TO_IDX(entry->offset);
	size = atop(entry->end - entry->start);

	/*
	 * If swap_pager_copy() is later called, it will convert new_object
	 * into a swap object.
	 */
	new_object = vm_object_allocate_anon(size, orig_object,
	    orig_object->cred, ptoa(size));

	/*
	 * We must wait for the orig_object to complete any in-progress
	 * collapse so that the swap blocks are stable below.  The
	 * additional reference on backing_object by new object will
	 * prevent further collapse operations until split completes.
	 */
	VM_OBJECT_WLOCK(orig_object);
	vm_object_collapse_wait(orig_object);

	/*
	 * At this point, the new object is still private, so the order in
	 * which the original and new objects are locked does not matter.
	 */
	VM_OBJECT_WLOCK(new_object);
	new_object->domain = orig_object->domain;
	backing_object = orig_object->backing_object;
	if (backing_object != NULL) {
		vm_object_backing_insert_ref(new_object, backing_object);
		new_object->backing_object_offset = 
		    orig_object->backing_object_offset + entry->offset;
	}
	if (orig_object->cred != NULL) {
		crhold(orig_object->cred);
		KASSERT(orig_object->charge >= ptoa(size),
		    ("orig_object->charge < 0"));
		orig_object->charge -= ptoa(size);
	}

	/*
	 * Mark the split operation so that swap_pager_getpages() knows
	 * that the object is in transition.