Shared memory is a way to shared state between process. Shared memory, as its name implies, is a method to “share” data between processes. Both processes define the same memory area as “shared”, and they can then exchange information simply by writing into it. This (used to be, and still is somewhat) faster than the alternative of sending network or pipe-based messages between processes.
If you see the memory as a mean of storing data, a file on a file system can be seen as shared memory (ie shared file). It is difficult to account for shared memory. Does it belong to one process? Both? Neither? If we naively sum the memory belonging to multiple processes, we grossly “over-count”.
As the name implies, the Shared (Virtual) Memory refers to virtual memory that are shared by more than one process and then can be used by multiple programs simultaneously. Although virtual memory allows processes to have separate (virtual) address spaces, there are times when you need processes to share memory. Shared memory (SHM) is another method of interprocess communication (IPC) whereby several processes share a single chunk of memory to communicate.
Shared memory provides the fastest way for processes to pass large amounts of data to one another. /dev/shm is nothing but implementation of traditional shared memory concept. It is an efficient means of passing data between programs. One program will create a memory portion, which other processes (if permitted) can access. This will result into speeding up things on Linux.
The Problem
On every reboot of the server, the /dev/shm permission changes:
$ ls -alrt /dev/ | grep shm drwxr-xr-t. 2 root root 60 jul 6 11:14 shm
Original Permission(1777):
# ls -ld /dev/shm drwxrwxrwt. 2 root root 200 Aug 20 03:44 /dev/shm
Existing Permission(1754):
$ ls -alrt /dev/ | grep shm drwxr-xr-t. 2 root root 60 jul 6 11:14 shm
The Solution
The cause of the issue is with the existing initscripts package [initscripts-9.49.37-1.0.1.el7_3.1.x86_64].
Workaround
Step 1: Mask the service( rhel-import-state):
# systemctl mask rhel-import-state
Step 2: Check the status of the service. It will look similar to below:
rhel-import-state.service Loaded: masked (/dev/null; bad)
Step 3: Reboot the machine and confirm whether the same issue reoccurs or not.
Reverting the Changes
You can also revert the changes by executing the below command to unmask the masked service.
# systemctl unmask rhel-import-state.service # systemctl status rhel-import-state.service