I'm just wondering where these values are being set and what they default to? Mine is currently 18446744073692774399. I didn't set it anywhere that I can see.
$ cat /proc/sys/kernel/shmmax 18446744073692774399 $ sysctl kernel.shmmax kernel.shmmax = 18446744073692774399 The __init function ipc_ns_init sets the initial value of shmmax by calling shm_init_ns, which sets it to the value of the SHMMAX macro.
The definition of SHMMAX is in <uapi/linux/shm.h>:
#define SHMMAX (ULONG_MAX - (1UL << 24)) /* max shared seg size (bytes) */ On 64-bit machines, that definition equals the value you found, 18446744073692774399.
On the fly you can just echo into /proc/sys/kernel/shmmax!
# echo 20446744073692774399 > /proc/sys/kernel/shmmax But most people edit /etc/sysctl.conf with a line similar to:
kernel.shmmax=your_new_value_here See the sysctl(8) man page.
