From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ob0-f172.google.com (mail-ob0-f172.google.com [209.85.214.172]) by kanga.kvack.org (Postfix) with ESMTP id A45116B0031 for ; Sat, 19 Apr 2014 02:55:21 -0400 (EDT) Received: by mail-ob0-f172.google.com with SMTP id wo20so2573768obc.3 for ; Fri, 18 Apr 2014 23:55:21 -0700 (PDT) Received: from g4t3425.houston.hp.com (g4t3425.houston.hp.com. [15.201.208.53]) by mx.google.com with ESMTPS id ny4si24429592obb.164.2014.04.18.23.55.20 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Fri, 18 Apr 2014 23:55:21 -0700 (PDT) Message-ID: <1397890512.19331.21.camel@buesod1.americas.hpqcorp.net> Subject: Re: [PATCH] ipc/shm: Increase the defaults for SHMALL, SHMMAX to infinity From: Davidlohr Bueso Date: Fri, 18 Apr 2014 23:55:12 -0700 In-Reply-To: <1397812720-5629-1-git-send-email-manfred@colorfullife.com> References: <1397812720-5629-1-git-send-email-manfred@colorfullife.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Manfred Spraul Cc: Andrew Morton , Davidlohr Bueso , LKML , KAMEZAWA Hiroyuki , KOSAKI Motohiro , gthelen@google.com, aswin@hp.com, linux-mm@kvack.org, mtk.manpages@gmail.com On Fri, 2014-04-18 at 11:18 +0200, Manfred Spraul wrote: > System V shared memory > > a) can be abused to trigger out-of-memory conditions and the standard > measures against out-of-memory do not work: > > - it is not possible to use setrlimit to limit the size of shm segments. > > - segments can exist without association with any processes, thus > the oom-killer is unable to free that memory. > > b) is typically used for shared information - today often multiple GB. > (e.g. database shared buffers) > > The current default is a maximum segment size of 32 MB and a maximum total > size of 8 GB. This is often too much for a) and not enough for b), which > means that lots of users must change the defaults. > > This patch increases the default limits to ULONG_MAX, which is perfect for > case b). The defaults are used after boot and as the initial value for > each new namespace. > > Admins/distros that need a protection against a) should reduce the limits > and/or enable shm_rmid_forced. > > Further notes: > - The patch only changes the boot time default, overrides behave as before: > # sysctl kernel/shmall=33554432 > would recreate the previous limit for SHMMAX (for the current namespace). > > - Disabling sysv shm allocation is possible with: > # sysctl kernel.shmall=0 > (not a new feature, also per-namespace) > > - ULONG_MAX is not really infinity, but 18 Exabyte segment size and > 75 Zettabyte total size. This should be enough for the next few weeks. > (assuming a 64-bit system with 4k pages) > > Risks: > - The patch breaks installations that use "take current value and increase > it a bit". [seems to exist, http://marc.info/?l=linux-mm&m=139638334330127] This really scares me. The probability of occurrence is now much higher, and not just theoretical. It would legitimately break userspace. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org