Add notes for section 12.7.

12.md

@@ -496,6 +496,26 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
   function then decrements the parent directory `struct inode->i_nlink` count
   and calls [shmem_unlink()][shmem_unlink] to remove it.
 
+## 12.7 Setting Up Shared Regions
+
+* A shared region is backed by a file created in `shm`.
+
+* There are two cases where a new file will be created - during the setup of a
+  shared region via (userland) [shmget()][shmget] or when an anonymous region is
+  set up via (userland) [mmap()][mmap] with the `MAP_SHARED` flag specified.
+
+* Both approaches ultimately use [shmem_file_setup()][shmem_file_setup] to
+  create a file, which simply creates a new [struct dentry][dentry] and
+  [struct inode][inode], fills in the relevant details and instantiates them.
+
+* Because the file system is internal, the names of the files created do _not_
+  have to be unique as the files are always located by inode, not name. As a
+  result, [shmem_zero_setup()][shmem_zero_setup] always creates a file
+  `"dev/zero"` which is how it shows up in `/proc/<pid>/maps`.
+
+* Files created by `shmget()` are named `SYSV<NN>` where `<NN>` is the key that
+  is passed as a parameter to `shmget()`.
+
 [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
@@ -567,3 +587,6 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
 [shmem_rmdir]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1232
 [shmem_empty]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1201
 [shmem_unlink]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1221
+
+[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