Function vm_fault_cow

freebsd/vm/vm_fault.c:892–996

}

static void
vm_fault_cow(struct faultstate *fs)
{
	bool is_first_object_locked;

	KASSERT(fs->object != fs->first_object,
	    ("source and target COW objects are identical"));

	/*
	 * This allows pages to be virtually copied from a backing_object
	 * into the first_object, where the backing object has no other
	 * refs to it, and cannot gain any more refs.  Instead of a bcopy,
	 * we just move the page from the backing object to the first
	 * object.  Note that we must mark the page dirty in the first
	 * object so that it will go out to swap when needed.
	 */
	is_first_object_locked = false;
	if (
	    /*
	     * Only one shadow object and no other refs.
	     */
	    fs->object->shadow_count == 1 && fs->object->ref_count == 1 &&
	    /*
	     * No other ways to look the object up
	     */
	    fs->object->handle == NULL && (fs->object->flags & OBJ_ANON) != 0 &&
	    /*
	     * We don't chase down the shadow chain and we can acquire locks.
	     */
	    (is_first_object_locked = VM_OBJECT_TRYWLOCK(fs->first_object)) &&
	    fs->object == fs->first_object->backing_object &&
	    VM_OBJECT_TRYWLOCK(fs->object)) {
		/*
		 * Remove but keep xbusy for replace.  fs->m is moved into
		 * fs->first_object and left busy while fs->first_m is
		 * conditionally freed.
		 */
		vm_page_remove_xbusy(fs->m);
		vm_page_replace(fs->m, fs->first_object, fs->first_pindex,
		    fs->first_m);
		vm_page_dirty(fs->m);
#if VM_NRESERVLEVEL > 0
		/*
		 * Rename the reservation.
		 */
		vm_reserv_rename(fs->m, fs->first_object, fs->object,
		    OFF_TO_IDX(fs->first_object->backing_object_offset));
#endif
		VM_OBJECT_WUNLOCK(fs->object);
		VM_OBJECT_WUNLOCK(fs->first_object);
		fs->first_m = fs->m;
		fs->m = NULL;
		VM_CNT_INC(v_cow_optim);
	} else {
		if (is_first_object_locked)
			VM_OBJECT_WUNLOCK(fs->first_object);
		/*
		 * Oh, well, lets copy it.