linux-kernel - [PATCH mmotm 4/5] nilfs2: insert explanations in gcinode file

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Date:	Fri, 12 Dec 2008 14:17:00 +0900
From:	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
	Joern Engel <joern@...fs.org>
Subject: [PATCH mmotm 4/5] nilfs2: insert explanations in gcinode file

The file gcinode.c gives buffer cache functions for on-disk blocks
moved in garbage collection.  Joern Engel has suggested inserting its
explanations in the source file (Message-ID:
<20080917144146.GD8750@...fs.org> and
<20080917224953.GB14644@...fs.org>).

This follows the comment.

Cc: Joern Engel <joern@...fs.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>
---
 fs/nilfs2/gcinode.c |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/fs/nilfs2/gcinode.c b/fs/nilfs2/gcinode.c
index 0013952..77615aa 100644
--- a/fs/nilfs2/gcinode.c
+++ b/fs/nilfs2/gcinode.c
@@ -1,5 +1,5 @@
 /*
- * 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.
  *
@@ -22,6 +22,25 @@
  * Revised by Ryusuke Konishi <ryusuke@...g.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>
-- 
1.5.6.5

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