Add notes for section 12.6.

12.md

@@ -448,6 +448,54 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
    returns 0 for success - because the files only exist in RAM, they don't need
    to be synchronised with any disk.
 
+## 12.6 Inode Operations in tmpfs
+
+* The most complex operation that is supported for inodes is truncation and
+  involves four distinct stages:
+
+1. [shmem_truncate()][shmem_truncate] - truncates a partial page at the end of
+   the file and continually calls
+   [shmem_truncate_indirect()][shmem_truncate_indirect] until the file is
+   truncated to the proper size. Each call to `shmem_truncate_indirect()` will
+   only process one indirect block at each pass, which is why it may need to be
+   called multiple times.
+
+2. [shmem_truncate_indirect()][shmem_truncate_indirect] - Deals with both doubly
+   and triply-indrect blocks. It finds the next indirect block that needs to be
+   truncated to pass to stage 3, and will contain pointers to pages which will
+   in turn contain swap vectors.
+
+3. [shmem_truncate_direct()][shmem_truncate_direct] - Selects a range of swap
+   vectors from a page that need to be truncated and passes that range to
+   [shmem_swp_free()][shmem_swp_free].
+
+4. [shmem_free_swp()][shmem_free_swp] - Frees entries via
+   [free_swap_and_cache()][free_swap_and_cache] which frees both the swap entry
+   and the page containing data.
+
+* The linking and unlinking of files is very simple because most of the work is
+  performed by the filesystem layer.
+
+* To link a file, the directory inode size is incremented, and `ctime` and
+  `mtime` of the affected inodes are updated and the number of links to the
+  inode being linked is incremented. A reference to the new
+  [struct dentry][dentry] is then created via [dget()][dget] and
+  [d_instantiate()][d_instantiate].
+
+* To unlink a file the same inode statistics are updated before decrementing the
+  references to the `struct dentry` only using [dput()][dput] (and subsequently
+  [iput()][iput]) which will clear up the inode when its reference count hits
+  zero.
+
+* Creating a directory will use [shmem_mkdir()][shmem_mkdir] which in turn uses
+  [shmem_mknod()][shmem_mknod] with the `S_IFDIR` flag set, before incrementing
+  the parent directory [struct inode][inode]'s `i_nlink` counter.
+
+* The function [shmem_rmdir()][shmem_rmdir] will delete a directory, first
+  making sure it's empty via [shmem_empty()][shmem_empty]. If it is, the
+  function then decrements the parent directory `struct inode->i_nlink` count
+  and calls [shmem_unlink()][shmem_unlink] to remove it.
+
 [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
@@ -505,3 +553,17 @@ ENTRIES_PER_PAGE/2 | |-- - -- - -- - -- - |   |  --------------- | |     -------
 [shmem_file_read]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1088
 [shmem_file_write]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L924
 [shmem_sync_file]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1446
+
+[shmem_truncate_indirect]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L309
+[shmem_truncate_direct]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L265
+[shmem_free_swp]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L240
+[free_swap_and_cache]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/swapfile.c#L332
+[dentry]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/dcache.h#L67
+[dget]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/dcache.h#L244
+[d_instantiate]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/fs/dcache.c#L651
+[dput]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/include/linux/dcache.h#L268
+[iput]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/fs/inode.c#L1027
+[shmem_mkdir]:https://github.com/lorenzo-stoakes/linux-historical/blob/v2.4.22/mm/shmem.c#L1154
+[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