[PATCH] ext2/3 commentary and cleanup

- Add some (much-needed) commentary to the ext2/ext3 block allocator
  state fields.

- Remove the SEARCH_FROM_ZERO debug code.  I wrote that to trigger
  some race and it hasn't been used in a year.

fs/ext2/ext2.h

@@ -14,8 +14,29 @@ struct ext2_inode_info {
 	__u32	i_file_acl;
 	__u32	i_dir_acl;
 	__u32	i_dtime;
+
+	/*
+	 * i_block_group is the number of the block group which contains
+	 * this file's inode.  Constant across the lifetime of the inode,
+	 * it is ued for making block allocation decisions - we try to
+	 * place a file's data blocks near its inode block, and new inodes
+	 * near to their parent directory's inode.
+	 */
 	__u32	i_block_group;
+
+	/*
+	 * i_next_alloc_block is the logical (file-relative) number of the
+	 * most-recently-allocated block in this file.  Yes, it is misnamed.
+	 * We use this for detecting linearly ascending allocation requests.
+	 */
 	__u32	i_next_alloc_block;
+
+	/*
+	 * i_next_alloc_goal is the *physical* companion to i_next_alloc_block.
+	 * it the the physical block number of the block which was most-recently
+	 * allocated to this file.  This give us the goal (target) for the next
+	 * allocation when we detect linearly ascending requests.
+	 */
 	__u32	i_next_alloc_goal;
 	__u32	i_prealloc_block;
 	__u32	i_prealloc_count;

fs/ext3/inode.c

@@ -39,13 +39,6 @@
 #include "xattr.h"
 #include "acl.h"
 
-/*
- * SEARCH_FROM_ZERO forces each block allocation to search from the start
- * of the filesystem.  This is to force rapid reallocation of recently-freed
- * blocks.  The file fragmentation is horrendous.
- */
-#undef SEARCH_FROM_ZERO
-
 /*
  * Test whether an inode is a fast symlink.
  */
@@ -510,10 +503,6 @@ static int ext3_find_goal(struct inode *inode, long block, Indirect chain[4],
 		ei->i_next_alloc_block++;
 		ei->i_next_alloc_goal++;
 	}
-#ifdef SEARCH_FROM_ZERO
-	ei->i_next_alloc_block = 0;
-	ei->i_next_alloc_goal = 0;
-#endif
 	/* Writer: end */
 	/* Reader: pointers, ->i_next_alloc* */
 	if (verify_chain(chain, partial)) {
@@ -525,9 +514,6 @@ static int ext3_find_goal(struct inode *inode, long block, Indirect chain[4],
 			*goal = ei->i_next_alloc_goal;
 		if (!*goal)
 			*goal = ext3_find_near(inode, partial);
-#ifdef SEARCH_FROM_ZERO
-		*goal = 0;
-#endif
 		return 0;
 	}
 	/* Reader: end */
@@ -673,10 +659,6 @@ static int ext3_splice_branch(handle_t *handle, struct inode *inode, long block,
 	*where->p = where->key;
 	ei->i_next_alloc_block = block;
 	ei->i_next_alloc_goal = le32_to_cpu(where[num-1].key);
-#ifdef SEARCH_FROM_ZERO
-	ei->i_next_alloc_block = 0;
-	ei->i_next_alloc_goal = 0;
-#endif
 	/* Writer: end */
 
 	/* We are done with atomic stuff, now do the rest of housekeeping */

include/linux/ext3_fs_i.h

@@ -32,9 +32,30 @@ struct ext3_inode_info {
 	__u32	i_file_acl;
 	__u32	i_dir_acl;
 	__u32	i_dtime;
+
+	/*
+	 * i_block_group is the number of the block group which contains
+	 * this file's inode.  Constant across the lifetime of the inode,
+	 * it is ued for making block allocation decisions - we try to
+	 * place a file's data blocks near its inode block, and new inodes
+	 * near to their parent directory's inode.
+	 */
 	__u32	i_block_group;
 	__u32	i_state;		/* Dynamic state flags for ext3 */
+
+	/*
+	 * i_next_alloc_block is the logical (file-relative) number of the
+	 * most-recently-allocated block in this file.  Yes, it is misnamed.
+	 * We use this for detecting linearly ascending allocation requests.
+	 */
 	__u32	i_next_alloc_block;
+
+	/*
+	 * i_next_alloc_goal is the *physical* companion to i_next_alloc_block.
+	 * it the the physical block number of the block which was most-recently
+	 * allocated to this file.  This give us the goal (target) for the next
+	 * allocation when we detect linearly ascending requests.
+	 */
 	__u32	i_next_alloc_goal;
 #ifdef EXT3_PREALLOCATE
 	__u32	i_prealloc_block;