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From:   David Howells <dhowells@redhat.com>
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To:     Al Viro <viro@zeniv.linux.org.uk>
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Subject: Re: [PATCH 1/2] open: add close_range()
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Date:   Tue, 21 May 2019 17:30:27 +0100
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Al Viro <viro@zeniv.linux.org.uk> 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