Add notes for section 12.3.

12.md

@@ -259,6 +259,37 @@ static struct inode_operations shmem_symlink_inode_operations = {
   sees no difference between pages backed by a real file and those backed by
   virtual files.
 
+## 12.3 Creating files in tmpfs
+
+* Because `tmpfs` is mounted as a proper filesystem that is visible to the user
+  it must support directory inode operations, as provided for by
+  [shmem_dir_inode_operations][shmem_dir_inode_operations], discussed in the
+  previous section.
+
+* The implementation of most of the functions used by
+  `shmem_dir_inode_operations` are rather small but they are all very
+  interconnected.
+
+* The majority of the inode fields provided by the VFS are filled by
+  [shmem_get_inode()][shmem_get_inode].
+
+* When creating a new file [shmem_create()][shmem_create] is used - this is a
+  small wrapper around [shmem_mknod()][shmem_mknod] which is called with the
+  `S_IFREG` flag set so a regular file will be created.
+
+* Again `shmem_mknod()` is really just a wrapper around `shmem_get_inode()`
+  which creates an inode and fills in the [struct inode][inode] fields as
+  required.
+
+* The 3 most interesting fields that are populated are
+  `inode->i_mapping->a_ops`, `inode->i_op` and `inode->i_fop` fields.
+
+* After the inode has been created, `shmem_mknod()` updates the directory
+  inode's `size` and `mtime` statistics before creating the new inode.
+
+* Files are created differently in `shm` despite the fact the filesystems are
+  basically the same - this is looked at in more detail in 12.7.
+
 [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
@@ -297,3 +328,8 @@ static struct inode_operations shmem_symlink_inode_operations = {
 [shmem_truncate]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L351
 [shmem_notify_change]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L375
 [shmem_getpage]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L583
+
+[shmem_get_inode]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L809
+[shmem_create]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1164
+[shmem_mknod]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1139
+[inode]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/fs.h#L438