[RFC] exclude bitmap and 32bit bitmap cheksum fields

[Amir Goldstein <amir73il@gmail.com>]

Hi Ted,

Following our conversation, here is a proposal how to squeeze:
- 32bit exclude bitmap block address
- 32bit block+exclude bitmap checksum
- 32bit inode bitmap checksum
into the reaming 8 bytes in the group descriptor.

The idea is that the 16bit persistent free inode/block counters
are redundant to the inode/block bitmap information
and are needed in 2 use cases:
1. sanity checks on fsck
2. quick load of in-memory counters

The first use case is nulled by the introduction of inode/block bitmap
checksums.
The second use case can be bypassed with no substantial penalty:
in-memory counters can be calculated on first inode/block bitmap access,
when the GRP_NEED_INIT (or another) flag is set in the group_info struct,
just like their cousins, the buddy bitmap counters.

Amir.

diff --git a/lib/ext2fs/ext2_fs.h b/lib/ext2fs/ext2_fs.h
index 0deb554..5cbaeb2 100644
--- a/lib/ext2fs/ext2_fs.h
+++ b/lib/ext2fs/ext2_fs.h
@@ -153,11 +153,11 @@ struct ext2_group_desc
 	__u32	bg_block_bitmap;	/* Blocks bitmap block */
 	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
 	__u32	bg_inode_table;		/* Inodes table block */
-	__u16	bg_free_blocks_count;	/* Free blocks count */
-	__u16	bg_free_inodes_count;	/* Free inodes count */
+	__u32	bg_exclude_bitmap;	/* Exclude bitmap block */
 	__u16	bg_used_dirs_count;	/* Directories count */
 	__u16	bg_flags;
-	__u32	bg_reserved[2];
+	__u32	bg_block_bitmap_csum;	/* Blocks+exclude bitmap checksum */
+	__u32	bg_inode_bitmap_csum;	/* Inodes bitmap checksum */
 	__u16	bg_itable_unused;	/* Unused inodes count */
 	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
 };
@@ -170,18 +170,17 @@ struct ext4_group_desc
 	__u32	bg_block_bitmap;	/* Blocks bitmap block */
 	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
 	__u32	bg_inode_table;		/* Inodes table block */
-	__u16	bg_free_blocks_count;	/* Free blocks count */
-	__u16	bg_free_inodes_count;	/* Free inodes count */
+	__u32	bg_exclude_bitmap;	/* Exclude bitmap block */
 	__u16	bg_used_dirs_count;	/* Directories count */
 	__u16	bg_flags;
-	__u32	bg_reserved[2];
+	__u32	bg_block_bitmap_csum;	/* Blocks+exclude bitmap checksum */
+	__u32	bg_inode_bitmap_csum;	/* Inodes bitmap checksum */
 	__u16	bg_itable_unused;	/* Unused inodes count */
 	__u16	bg_checksum;		/* crc16(s_uuid+grouo_num+group_desc)*/
 	__u32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
 	__u32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
 	__u32	bg_inode_table_hi;	/* Inodes table block MSB */
-	__u16	bg_free_blocks_count_hi;/* Free blocks count MSB */
-	__u16	bg_free_inodes_count_hi;/* Free inodes count MSB */
+	__u32	bg_exclude_bitmap;	/* Exclude bitmap block MSB */
 	__u16	bg_used_dirs_count_hi;	/* Directories count MSB */
 	__u16   bg_pad;
 	__u32	bg_reserved2[3];
@@ -190,6 +189,7 @@ struct ext4_group_desc
 #define EXT2_BG_INODE_UNINIT	0x0001 /* Inode table/bitmap not initialized */
 #define EXT2_BG_BLOCK_UNINIT	0x0002 /* Block bitmap not initialized */
 #define EXT2_BG_INODE_ZEROED	0x0004 /* On-disk itable initialized to zero */
+#define EXT2_BG_EXCLUDE_UNINIT	0x0008 /* Exclude bitmap not initialized */

 /*
  * Data structures used by the directory indexing feature
@@ -751,6 +751,7 @@ struct ext2_super_block {
 #define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
 #define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040
 #define EXT4_FEATURE_RO_COMPAT_HAS_SNAPSHOT	0x0080
+#define EXT4_FEATURE_RO_COMPAT_BITMAP_CSUM	0x0100

 #define EXT2_FEATURE_INCOMPAT_COMPRESSION	0x0001
 #define EXT2_FEATURE_INCOMPAT_FILETYPE		0x0002