From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 1/2] open: add close_range()
Date: Tue, 21 May 2019 17:30:27 +0100
Message-ID: <28114.1558456227@warthog.procyon.org.uk>
References: <20190521150006.GJ17978@ZenIV.linux.org.uk> <20190521113448.20654-1-christian@brauner.io>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 8BIT
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20190521150006.GJ17978@ZenIV.linux.org.uk>
Content-ID: <28113.1558456227.1@warthog.procyon.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
List-Archive: <https://lore.kernel.org/linux-api/>
List-Post: <mailto:linux-api@vger.kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: dhowells@redhat.com, Christian Brauner <christian@brauner.io>, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, jannh@google.com, fweimer@redhat.com, oleg@redhat.com, tglx@linutronix.de, torvalds@linux-foundation.org, arnd@arndb.de, shuah@kernel.org, tkjos@android.com, ldv@altlinux.org, miklos@szeredi.hu, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, x86@kernel.org
List-ID: <linux-s390.vger.kernel.org>

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