Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752123AbaDQUYH (ORCPT ); Thu, 17 Apr 2014 16:24:07 -0400 Received: from mail-pb0-f43.google.com ([209.85.160.43]:48676 "EHLO mail-pb0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751041AbaDQUXy (ORCPT ); Thu, 17 Apr 2014 16:23:54 -0400 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <534FFFC2.6050601@colorfullife.com> References: <1397272942.2686.4.camel@buesod1.americas.hpqcorp.net> <534FFFC2.6050601@colorfullife.com> From: "Michael Kerrisk (man-pages)" Date: Thu, 17 Apr 2014 22:23:32 +0200 Message-ID: Subject: Re: [PATCH v2] ipc,shm: disable shmmax and shmall by default To: Manfred Spraul Cc: Davidlohr Bueso , Andrew Morton , KOSAKI Motohiro , Kamezawa Hiroyuki , Greg Thelen , aswin@hp.com, LKML , "linux-mm@kvack.org" Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Manfred! On Thu, Apr 17, 2014 at 6:22 PM, Manfred Spraul wrote: > Hi Michael, > > > On 04/17/2014 12:53 PM, Michael Kerrisk wrote: >> >> On Sat, Apr 12, 2014 at 5:22 AM, Davidlohr Bueso wrote: >>> >>> From: Davidlohr Bueso >>> >>> 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. >>> >>> This change allows Linux to treat shm just as regular anonymous memory. >>> One important difference between them, though, is handling out-of-memory >>> conditions: as opposed to regular anon memory, the OOM killer will not >>> free the memory as it is shm, allowing users to potentially abuse this. >>> To overcome this situation, the shm_rmid_forced option must be enabled. >>> >>> [1]: http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html >>> >>> Signed-off-by: Davidlohr Bueso >>> Acked-by: KAMEZAWA Hiroyuki >>> Acked-by: KOSAKI Motohiro >> >> Of the two proposed approaches (the other being >> marc.info/?l=linux-kernel&m=139730332306185), this looks preferable to >> me, since it allows strange users to maintain historical behavior >> (i.e., the ability to set a limit) if they really want it, so: >> >> Acked-by: Michael Kerrisk >> >> One or two comments below, that you might consider for your v3 patch. > > I don't understand what you mean. As noted in the other mail, you don't understand, because I was being dense (and misled a little by the commit message). > After a > # echo 33554432 > /proc/sys/kernel/shmmax > # echo 2097152 > /proc/sys/kernel/shmmax > > both patches behave exactly identical. Yes. > There are only two differences: > - Davidlohr's patch handles > # echo > > /proc/sys/kernel/shmmax > With my patch, shmmax would end up as 0 and all allocations fail. > > - My patch handles the case if some startup code/installer checks > shmmax and complains if it is below the requirement of the application. Thanks for that clarification. I withdraw my Ack. In fact, maybe I even like your approach a little more, because of that last point. Did one of you not yet manage to persuade the other to his point of view yet? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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/