From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1760789AbcAKXDT (ORCPT <rfc822;w@1wt.eu>);
	Mon, 11 Jan 2016 18:03:19 -0500
Received: from relay4-d.mail.gandi.net ([217.70.183.196]:50028 "EHLO
	relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753955AbcAKXDP (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 11 Jan 2016 18:03:15 -0500
Date: Mon, 11 Jan 2016 15:03:06 -0800
From: Josh Triplett <josh@joshtriplett.org>
To: "Seymour, Shane M" <shane.seymour@hpe.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Thomas Gleixner <tglx@linutronix.de>, Paul Turner <pjt@google.com>,
        Andrew Hunter <ahh@google.com>, Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Andi Kleen <andi@firstfloor.org>, Dave Watson <davejwatson@fb.com>,
        Chris Lameter <cl@linux.com>, Ingo Molnar <mingo@redhat.com>,
        Ben Maurer <bmaurer@fb.com>, Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [RFC PATCH 0/3] Implement getcpu_cache system call
Message-ID: <20160111230306.GC28717@cloud>
References: <1451977320-4886-1-git-send-email-mathieu.desnoyers@efficios.com>
 <DDB9C85B850785449757F9914A034FCB4450ACF7@G9W0766.americas.hpqcorp.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <DDB9C85B850785449757F9914A034FCB4450ACF7@G9W0766.americas.hpqcorp.net>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jan 11, 2016 at 10:38:28PM +0000, Seymour, Shane M wrote:
> I have some concerns and suggestions for you about this.
> 
> What's to stop someone in user space from requesting an arbitrarily large number of CPU # cache locations that the kernel needs to allocate memory to track and each time the task migrates to a new CPU it needs to update them all? Could you use it to dramatically slow down a system/task switching? Should there be a ulimit type value or a sysctl setting to limit the number that you're allowed to register per-task?

The documented behavior of the syscall allows only one location per
thread, so the kernel can track that one and only address rather easily
in the task_struct.  Allowing dynamic allocation definitely doesn't seem
like a good idea.

- Josh Triplett