Al Viro [off-list ref] wrote:

Umm...  That's going to be very painful if you dup2() something to MAX_INT and
then run that; roughly 2G iterations of bouncing ->file_lock up and down,
without anything that would yield CPU in process.

If anything, I would suggest something like

	fd = *start_fd;
	grab the lock
        fdt = files_fdtable(files);
more:
	look for the next eviction candidate in ->open_fds, starting at fd
	if there's none up to max_fd
		drop the lock
		return NULL
	*start_fd = fd + 1;
	if the fscker is really opened and not just reserved
		rcu_assign_pointer(fdt->fd[fd], NULL);
		__put_unused_fd(files, fd);
		drop the lock
		return the file we'd got
	if (unlikely(need_resched()))
		drop lock
		cond_resched();
		grab lock
		fdt = files_fdtable(files);
	goto more;

with the main loop being basically
	while ((file = pick_next(files, &start_fd, max_fd)) != NULL)
		filp_close(file, files);

If we can live with close_from(int first) rather than close_range(), then this
can perhaps be done a lot more efficiently by:

	new = alloc_fdtable(first);
	spin_lock(&files->file_lock);
	old = files_fdtable(files);
	copy_fds(new, old, 0, first - 1);
	rcu_assign_pointer(files->fdt, new);
	spin_unlock(&files->file_lock);
	clear_fds(old, 0, first - 1);
	close_fdt_from(old, first);
	kfree_rcu(old);

David