[PATCH v2] ipc,shm: document new limits in the uapi header

[Davidlohr Bueso <davidlohr@hp.com>]

This is useful in the future and allows users to
better understand the reasoning behind the changes.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
---
 include/uapi/linux/shm.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
index 74e786d..3400b6e 100644
--- a/include/uapi/linux/shm.h
+++ b/include/uapi/linux/shm.h
@@ -8,17 +8,20 @@
 #endif
 
 /*
- * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
- * be modified by sysctl.
+ * SHMMNI, SHMMAX and SHMALL are the default upper limits which can be
+ * modified by sysctl. Both SHMMAX and SHMALL have their default values
+ * to the maximum limit which is as large as it can be without helping
+ * userspace overflow the values. There is really nothing the kernel
+ * can do to avoid this any further. It is therefore not advised to
+ * make them any larger. These limits are suitable for both 32 and
+ * 64-bit systems.
  */
-
 #define SHMMIN 1			 /* min shared seg size (bytes) */
 #define SHMMNI 4096			 /* max num of segs system wide */
-#define SHMMAX (ULONG_MAX - (1L<<24))	 /* max shared seg size (bytes) */
-#define SHMALL (ULONG_MAX - (1L<<24))	 /* max shm system wide (pages) */
+#define SHMMAX (ULONG_MAX - (1UL << 24)) /* max shared seg size (bytes) */
+#define SHMALL (ULONG_MAX - (1UL << 24)) /* max shm system wide (pages) */
 #define SHMSEG SHMMNI			 /* max shared segs per process */
 
-
 /* Obsolete, used only for backwards compatibility and libc5 compiles */
 struct shmid_ds {
 	struct ipc_perm		shm_perm;	/* operation perms */
-- 
1.8.1.4