nilfs2: insert explanations in gcinode file

/*

 * gcinode.c - NILFS memory inode for GC

 * gcinode.c - dummy inodes to buffer blocks for garbage collection

 *

 * Copyright (C) 2005-2008 Nippon Telegraph and Telephone Corporation.

 *

 * Revised by Ryusuke Konishi <ryusuke@osrg.net>.

 *

 */

/*

 * This file adds the cache of on-disk blocks to be moved in garbage

 * collection.  The disk blocks are held with dummy inodes (called

 * gcinodes), and this file provides lookup function of the dummy

 * inodes and their buffer read function.

 *

 * Since NILFS2 keeps up multiple checkpoints/snapshots accross GC, it

 * has to treat blocks that belong to a same file but have different

 * checkpoint numbers.  To avoid interference among generations, dummy

 * inodes are managed separatly from actual inodes, and their lookup

 * function (nilfs_gc_iget) is designed to be specified with a

 * checkpoint number argument as well as an inode number.

 *

 * Buffers and pages held by the dummy inodes will be released each

 * time after they are copied to a new log.  Dirty blocks made on the

 * current generation and the blocks to be moved by GC never overlap

 * because the dirty blocks make a new generation; they rather must be

 * written individually.

 */

#include <linux/buffer_head.h>

#include <linux/mpage.h>