On Sat, Apr 12, 2014 at 5:22 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote: > From: Davidlohr Bueso <davidlohr@xxxxxx> > > 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 <davidlohr@xxxxxx> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> 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 <mtk.manpages@xxxxxxxxx> 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 <asm/page.h> > #include <uapi/linux/shm.h> > > -#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */ > +#define SHMALL 0 /* max shm system wide (pages) */ > #include <asm/shmparam.h> > 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@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>