Add notes for section 12.8.

12.md

@@ -516,6 +516,32 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
 * Files created by `shmget()` are named `SYSV<NN>` where `<NN>` is the key that
   is passed as a parameter to `shmget()`.
 
+## 12.8 System V IPC
+
+* To avoid spending too long on this, we focus only on [shmget()][shmget] and
+  [shmat()][shmat] and how they are affected by the VM.
+
+* `shmget()` is implemented via [sys_shmget()][sys_shmget]. It performs sanity
+  checks, sets up the IPC-related data structures and a new segment via
+  [newseg()][newseg] which creates the file in `shmfs` with
+  [shmem_file_setup()][shmem_file_setup] as previously discussed.
+
+* `shmat()` is implemented via [sys_shmat()][sys_shmat]. It does sanity checks,
+  acquires the appropriate descriptor and hands over to [do_mmap()][do_mmap] to
+  map the shared region into the process address space.
+
+* `sys_shmat()` is responsible for ensuring that VMAs don't overlap if the
+  caller erroneously specifies to do so.
+
+* The [struct shmid_kernel][shmid_kernel]`->shm_nattch` counter is maintained by
+  [shm_vm_ops][shm_vm_ops], a
+  [struct vm_operations_struct][vm_operations_struct] that registers `open()`
+  and `close()` callbacks of [shm_open()][shm_open] and [shm_close()][shm_close]
+  respectively.
+
+* `shm_close()` is also responsible for destroying shared regions if the
+  `SHM_DEST` flag is specified and the `shm_nattch` counter reaches zero.
+
 [mmap]:http://man7.org/linux/man-pages/man2/mmap.2.html
 [fork]:http://man7.org/linux/man-pages/man2/fork.2.html
 [shmget]:http://man7.org/linux/man-pages/man2/shmget.2.html
@@ -590,3 +616,12 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
 
 [shmem_file_setup]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1607
 [shmem_zero_setup]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1664
+
+[sys_shmget]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L229
+[newseg]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L178
+[sys_shmat]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L568
+[do_mmap]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/mm.h#L557
+[shmid_kernel]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L29
+[shm_vm_ops]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L172
+[shm_open]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L110
+[shm_close]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/ipc/shm.c#L139