Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753886AbaDCXkD (ORCPT ); Thu, 3 Apr 2014 19:40:03 -0400 Received: from mail-oa0-f54.google.com ([209.85.219.54]:40575 "EHLO mail-oa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753443AbaDCXj6 (ORCPT ); Thu, 3 Apr 2014 19:39:58 -0400 MIME-Version: 1.0 In-Reply-To: <1396554637.2550.11.camel@buesod1.americas.hpqcorp.net> References: <1396235199.2507.2.camel@buesod1.americas.hpqcorp.net> <20140331170546.3b3e72f0.akpm@linux-foundation.org> <1396371699.25314.11.camel@buesod1.americas.hpqcorp.net> <1396377083.25314.17.camel@buesod1.americas.hpqcorp.net> <1396386062.25314.24.camel@buesod1.americas.hpqcorp.net> <20140401142947.927642a408d84df27d581e36@linux-foundation.org> <20140401144801.603c288674ab8f417b42a043@linux-foundation.org> <1396394931.25314.34.camel@buesod1.americas.hpqcorp.net> <1396484447.2953.1.camel@buesod1.americas.hpqcorp.net> <533DB03D.7010308@colorfullife.com> <1396554637.2550.11.camel@buesod1.americas.hpqcorp.net> From: KOSAKI Motohiro Date: Thu, 3 Apr 2014 19:39:38 -0400 Message-ID: Subject: Re: [PATCH] ipc,shm: disable shmmax and shmall by default To: Davidlohr Bueso Cc: Manfred Spraul , Andrew Morton , aswin@hp.com, LKML , "linux-mm@kvack.org" , Greg Thelen , Kamezawa Hiroyuki Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 3, 2014 at 3:50 PM, Davidlohr Bueso wrote: > On Thu, 2014-04-03 at 21:02 +0200, Manfred Spraul wrote: >> Hi Davidlohr, >> >> On 04/03/2014 02:20 AM, Davidlohr Bueso wrote: >> > The default size for shmmax is, and always has been, 32Mb. >> > Today, in the XXI century, it seems that this value is rather small, >> > making users have to increase it via sysctl, which can cause >> > unnecessary work and userspace application workarounds[1]. >> > >> > Instead of choosing yet another arbitrary value, larger than 32Mb, >> > this patch disables the use of both shmmax and shmall by default, >> > allowing users to create segments of unlimited sizes. Users and >> > applications that already explicitly set these values through sysctl >> > are left untouched, and thus does not change any of the behavior. >> > >> > So a value of 0 bytes or pages, for shmmax and shmall, respectively, >> > implies unlimited memory, as opposed to disabling sysv shared memory. >> > This is safe as 0 cannot possibly be used previously as SHMMIN is >> > hardcoded to 1 and cannot be modified. > >> Are we sure that no user space apps uses shmctl(IPC_INFO) and prints a >> pretty error message if shmall is too small? >> We would break these apps. > > Good point. 0 bytes/pages would definitely trigger an unexpected error > message if users did this. But on the other hand I'm not sure this > actually is a _real_ scenario, since upon overflow the value can still > end up being 0, which is totally bogus and would cause the same > breakage. > > So I see two possible workarounds: > (i) Use ULONG_MAX for the shmmax default instead. This would make shmall > default to 1152921504606846720 and 268435456, for 64 and 32bit systems, > respectively. > > (ii) Keep the 0 bytes, but add a new a "transition" tunable that, if set > (default off), would allow 0 bytes to be unlimited. With time, users > could hopefully update their applications and we could eventually get > rid of it. This _seems_ to be the less aggressive way to go. Do you mean set 0: IPC_INFO return shmmax = 0. set 1: IPC_INFO return shmmax = ULONG_MAX. ? That makes sense. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/