Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752251AbaDRPhK (ORCPT ); Fri, 18 Apr 2014 11:37:10 -0400 Received: from mail-pa0-f42.google.com ([209.85.220.42]:55997 "EHLO mail-pa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751139AbaDRPhH (ORCPT ); Fri, 18 Apr 2014 11:37:07 -0400 MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: <5350EFAA.2030607@colorfullife.com> References: <1397784345.2556.26.camel@buesod1.americas.hpqcorp.net> <5350EFAA.2030607@colorfullife.com> From: "Michael Kerrisk (man-pages)" Date: Fri, 18 Apr 2014 17:36:46 +0200 Message-ID: Subject: Re: [PATCH v3] 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" , Linux API 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 On Fri, Apr 18, 2014 at 11:26 AM, Manfred Spraul wrote: > Hi Davidlohr, > > > On 04/18/2014 03:25 AM, Davidlohr Bueso wrote: >> >> So a value of 0 bytes or pages, for shmmax and shmall, respectively, >> implies unlimited memory, as opposed to disabling sysv shared memory. > > That might be a second risk: > Right now, a sysadmin can prevent sysv memory allocations with > > # sysctl kernel.shmall=0 > > After your patch is applied, this line allows unlimited allocations. Good point. I wonder if some folk may get bitten by this complete reversal the semantics of shmall==0. > Obviously my patch has the opposite problem: 64-bit wrap-arounds. I know you alluded to a case in another thread, but I couldn't quite work out from the mail you referred to whether this was really the problem. (And I assume those folks were forced to fix their set-up scripts anyway.) So, it's not clear to me whether this is a real problem. (And your patch does not worsen things from the current situation, right?) Cheers, Michael >> --- a/include/uapi/linux/shm.h >> +++ b/include/uapi/linux/shm.h >> @@ -9,14 +9,14 @@ >> /* >> * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can >> - * be increased by sysctl >> + * be modified by sysctl. By default, disable SHMMAX and SHMALL with >> + * 0 bytes, thus allowing processes to have unlimited shared memory. >> */ >> - >> -#define SHMMAX 0x2000000 /* max shared seg size (bytes) */ >> +#define SHMMAX 0 /* max shared seg size (bytes) */ >> #define SHMMIN 1 /* min shared seg size (bytes) */ >> #define SHMMNI 4096 /* max num of segs system wide */ >> #ifndef __KERNEL__ >> -#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16)) >> +#define SHMALL 0 >> #endif >> #define SHMSEG SHMMNI /* max shared segs per process */ >> > > The "#ifndef __KERNEL__" is not required: > As there is no reference to PAGE_SIZE anymore, one definition for SHMALL is > sufficient. > > > -- > Manfred -- 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/