[PATCH v3] ipc,shm: disable shmmax and shmall by default

[Davidlohr Bueso]

The default size for shmmax is, and always has been, 32Mb.
Today, this value is rather small, making users have to
increase it via sysctl, which can cause unnecessary work and
userspace application workarounds. Ie:

http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html

Unix has historically required setting these limits for shared
memory, and Linux inherited such behavior. The consequence of this
is added complexity for users and administrators. One very common
example are Database setup/installation documents and scripts,
where users must manually calculate the values for these limits.
This also requires (some) knowledge of how the underlying memory
management works, thus causing, in many occasions, the limits to
just be flat out wrong. Disabling these limits sooner could have
saved companies a lot of time, headaches and money for support.
But it's never too late, simplify users life now.

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 will of course
be reflected in shmctl(SHM_STAT) calls. Any application that does
preliminary checking of the size of shmmax, must also check for
shmmin, and therefore the kernel can safely make this change. It is
well stated that any sizes must be within both ranges.

Another advantage of setting these values to 0 is that we automatically
take care of any variable overflowing problems, where the limit can
accidentally become 0. Without this change, such situations are just
*broken*, where shmmax = 0 < shmmin = 1.

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.

Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
Changes from v2:
 - Improve changelog (per Andrew/Manfred).
 - Minor documentation updates (per Michael).

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      | 3 ++-
 include/uapi/linux/shm.h | 8 ++++----
 ipc/shm.c                | 6 ++++--
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/include/linux/shm.h b/include/linux/shm.h
index 1e2cd2e..34e6ba74 100644
--- a/include/linux/shm.h
+++ b/include/linux/shm.h
@@ -4,7 +4,8 @@
 #include <asm/page.h>
 #include <uapi/linux/shm.h>
 
-#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */
+/* max shm system wide (pages), 0 being unlimited */
+#define SHMALL 0
 #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..d645c0c 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 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 */
 
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



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