This parameter sets the total amount of shared memory pages that can be used inside ipc namespace. The shared memory pages counting occurs for each ipc namespace separately and is not inherited. Hence, shmall should always be at least ceil(shmmax/PAGE_SIZE).
If you are not sure what the default PAGE_SIZE is on your Linux system, you can run the following command::
# getconf PAGE_SIZE
To reduce or disable the ability to allocate shared memory, you must create a new ipc namespace, set this parameter to the required value and prohibit the creation of a new ipc namespace in the current user namespace or cgroups can be used.