Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161340AbaDQKyT (ORCPT ); Thu, 17 Apr 2014 06:54:19 -0400 Received: from mail-we0-f176.google.com ([74.125.82.176]:47686 "EHLO mail-we0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751428AbaDQKyN (ORCPT ); Thu, 17 Apr 2014 06:54:13 -0400 MIME-Version: 1.0 In-Reply-To: <1397272942.2686.4.camel@buesod1.americas.hpqcorp.net> References: <1397272942.2686.4.camel@buesod1.americas.hpqcorp.net> From: Michael Kerrisk Date: Thu, 17 Apr 2014 12:53:52 +0200 X-Google-Sender-Auth: Dyl1_fCyF71756htykxonOeCPvY Message-ID: Subject: Re: [PATCH v2] ipc,shm: disable shmmax and shmall by default To: Davidlohr Bueso Cc: Andrew Morton , Manfred Spraul , KOSAKI Motohiro , Kamezawa Hiroyuki , Greg Thelen , aswin@hp.com, LKML , "linux-mm@kvack.org" , Michael Kerrisk-manpages 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 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. > --- > Changes from v1: > - Respect SHMMIN even when shmmax is 0 (unlimited). > This fixes the shmget02 test that broke in v1. (per Manfred) > > - Update changelog regarding OOM description (per Kosaki) > > include/linux/shm.h | 2 +- > include/uapi/linux/shm.h | 8 ++++---- > ipc/shm.c | 6 ++++-- > 3 files changed, 9 insertions(+), 7 deletions(-) > > diff --git a/include/linux/shm.h b/include/linux/shm.h > index 1e2cd2e..0ca06a3 100644 > --- a/include/linux/shm.h > +++ b/include/linux/shm.h > @@ -4,7 +4,7 @@ > #include > #include > > -#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */ > +#define SHMALL 0 /* max shm system wide (pages) */ > #include > struct shmid_kernel /* private to the kernel */ > { > diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h > index 78b6941..5f0ef28 100644 > --- 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 increased by sysctl. By default, disable SHMMAX and SHMALL with s/increased/modified/ > + * 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) */ I suggest: s/(bytes)/(bytes); 0 means "no limit" */ > #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 As long as we're here, let's add a meaningful comment to that one: /* system-wide limit on number of pages of shared memory; 0 means "no limit" */ Cheers, Michael > #endif > #define SHMSEG SHMMNI /* max shared segs per process */ > > diff --git a/ipc/shm.c b/ipc/shm.c > index 7645961..8630561 100644 > --- a/ipc/shm.c > +++ b/ipc/shm.c > @@ -490,10 +490,12 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params) > int id; > vm_flags_t acctflag = 0; > > - if (size < SHMMIN || size > ns->shm_ctlmax) > + if (size < SHMMIN || > + (ns->shm_ctlmax && size > ns->shm_ctlmax)) > return -EINVAL; > > - if (ns->shm_tot + numpages > ns->shm_ctlall) > + if (ns->shm_ctlall && > + ns->shm_tot + numpages > ns->shm_ctlall) > return -ENOSPC; > > shp = ipc_rcu_alloc(sizeof(*shp)); > -- > 1.8.1.4 > > > > -- > 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 -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Author of "The Linux Programming Interface", http://blog.man7.org/ -- 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/