[PATCH] Ext4: Uninitialized Block Groups

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from: Andreas Dilger <[email protected]>
In pass1 of e2fsck, every inode table in the fileystem is scanned and checked, regardless of whether it is in use. This is this the most time consuming part of the filesystem check. The unintialized block group feature can greatly reduce e2fsck time by eliminating checking of uninitialized inodes. With this feature, there is a a high water mark of used inodes for each block group. Block and inode bitmaps can be uninitialized on disk via a flag in the 
group descriptor to avoid reading or scanning them at e2fsck time.  A checksum
of each group descriptor is used to ensure that corruption in the group
descriptor's bit flags does not cause incorrect operation.

The feature is enabled through a mkfs option

	mke2fs /dev/ -O uninit_groups
A patch adding support for uninitialized block groups to e2fsprogs tools has been posted to the linux-ext4 mailing list.
The patches have been stress tested with fsstress and fsx. In performance tests testing e2fsck time, we have seen that e2fsck time on ext3 grows linearly with the total number of inodes in the filesytem. In ext4 with the uninitialized block groups feature, the e2fsck time is constant, based solely on the number of used inodes rather than the total inode count. Since typical ext4 filesystems only use 1-10% of their inodes, this feature can greatly reduce e2fsck time for users. With performance improvement of 2-20 times, depending on how full the filesystem is. 

The attached graph shows the major improvements in e2fsck times in filesystems
with a large total inode count, but few inodes in use. 
In each group descriptor if we have

EXT4_BG_INODE_UNINIT set in bg_flags:
       Inode table is not initialized/used in this group. So we can skip
       the consistency check during fsck.
EXT4_BG_BLOCK_UNINIT set in bg_flags:
       No block in the group is used. So we can skip the block bitmap
       verification for this group.

We also add two new fields to group descriptor as a part of
uninitialized group patch.

       __le16  bg_itable_unused;       /* Unused inodes count */
       __le16  bg_checksum;            /* crc16(sb_uuid+group+desc) */


bg_itable_unused:

If we have EXT4_BG_INODE_UNINIT not set in bg_flags
then bg_itable_unused will give the offset within
the inode table till the inodes are used. This can be
used by fsck to skip list of inodes that are marked unused.


bg_checksum:
Now that we depend on bg_flags and bg_itable_unused to determine
the block and inode usage, we need to make sure group descriptor
is not corrupt. We add checksum to group descriptor to
detect corruption. If the descriptor is found to be corrupt, we
mark all the blocks and inodes in the group used.


Signed-off-by: Andreas Dilger <[email protected]>
Signed-off-by: Avantika Mathur <[email protected]>
Signed-off-by: Mingming Cao <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>

---
fs/ext4/balloc.c        |   94 +++++++++++++++++++++++++++++++++-
fs/ext4/group.h         |   29 ++++++++++
fs/ext4/ialloc.c        |  132 ++++++++++++++++++++++++++++++++++++++++++++----
fs/ext4/resize.c | 2 fs/ext4/super.c | 96 ++++++++++++++++++++++++++++++++++ 
include/linux/ext4_fs.h |   14 ++++-
6 files changed, 352 insertions(+), 15 deletions(-)

Index: linux-2.6.23-rc6/fs/ext4/balloc.c
===================================================================
--- linux-2.6.23-rc6.orig/fs/ext4/balloc.c	2007-09-10 19:50:29.000000000 -0700
+++ linux-2.6.23-rc6/fs/ext4/balloc.c	2007-09-18 11:38:24.000000000 -0700
@@ -20,6 +20,7 @@
#include <linux/quotaops.h>
#include <linux/buffer_head.h>

+#include "group.h"
/*
 * balloc.c contains the blocks allocation and deallocation routines
 */
@@ -42,6 +43,74 @@

}

+/* Initializes an uninitialized block bitmap if given, and returns the
+ * number of blocks free in the group. */
+unsigned ext4_init_block_bitmap(struct super_block *sb, struct buffer_head *bh,
+				int block_group, struct ext4_group_desc *gdp)
+{
+	unsigned long start;
+	int bit, bit_max;
+	unsigned free_blocks;
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+
+	if (bh) {
+		J_ASSERT_BH(bh, buffer_locked(bh));
+
+		/* If checksum is bad mark all blocks used to prevent allocation
+		 * essentially implementing a per-group read-only flag. */
+		if (!ext4_group_desc_csum_verify(sbi, block_group, gdp)) {
+			ext4_error(sb, __FUNCTION__,
+				   "Checksum bad for group %u\n", block_group);
+			gdp->bg_free_blocks_count = 0;
+			gdp->bg_free_inodes_count = 0;
+			gdp->bg_itable_unused = 0;
+			memset(bh->b_data, 0xff, sb->s_blocksize);
+			return 0;
+		}
+		memset(bh->b_data, 0, sb->s_blocksize);
+	}
+
+	/* Check for superblock and gdt backups in this group */
+	bit_max = ext4_bg_has_super(sb, block_group);
+
+	if (!EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_META_BG) ||
+	    block_group < le32_to_cpu(sbi->s_es->s_first_meta_bg) *
+			  sbi->s_desc_per_block) {
+		if (bit_max) {
+			bit_max += ext4_bg_num_gdb(sb, block_group);
+			bit_max +=
+				le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks);
+		}
+	} else { /* For META_BG_BLOCK_GROUPS */
+		int group_rel = (block_group -
+				 le32_to_cpu(sbi->s_es->s_first_meta_bg)) %
+				EXT4_DESC_PER_BLOCK(sb);
+		if (group_rel == 0 || group_rel == 1 ||
+		    (group_rel == EXT4_DESC_PER_BLOCK(sb) - 1))
+			bit_max += 1;
+	}
+
+	/* Last and first groups are always initialized */
+	free_blocks = EXT4_BLOCKS_PER_GROUP(sb) - bit_max;
+
+	if (bh) {
+		for (bit = 0; bit < bit_max; bit++)
+			ext4_set_bit(bit, bh->b_data);
+
+		start = block_group * EXT4_BLOCKS_PER_GROUP(sb) +
+			le32_to_cpu(sbi->s_es->s_first_data_block);
+
+		/* Set bits for block and inode bitmaps, and inode table */
+		ext4_set_bit(ext4_block_bitmap(sb, gdp) - start, bh->b_data);
+		ext4_set_bit(ext4_inode_bitmap(sb, gdp) - start, bh->b_data);
+		for (bit = le32_to_cpu(gdp->bg_inode_table) - start,
+		     bit_max = bit + sbi->s_itb_per_group; bit < bit_max; bit++)
+			ext4_set_bit(bit, bh->b_data);
+	}
+
+	return free_blocks - sbi->s_itb_per_group - 2;
+}
+
/*
 * The free blocks are managed by bitmaps.  A file system contains several
 * blocks groups.  Each group contains 1 bitmap block for blocks, 1 bitmap
@@ -110,16 +179,29 @@
 *
 * Return buffer_head on success or NULL in case of failure.
 */
-static struct buffer_head *
+struct buffer_head *
read_block_bitmap(struct super_block *sb, unsigned int block_group)
{
	struct ext4_group_desc * desc;
	struct buffer_head * bh = NULL;

-	desc = ext4_get_group_desc (sb, block_group, NULL);
+	desc = ext4_get_group_desc(sb, block_group, NULL);
	if (!desc)
		goto error_out;
-	bh = sb_bread(sb, ext4_block_bitmap(sb, desc));
+	if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
+		bh = sb_getblk(sb, ext4_block_bitmap(sb, desc));
+		if (!buffer_uptodate(bh)) {
+			lock_buffer(bh);
+			if (!buffer_uptodate(bh)) {
+				ext4_init_block_bitmap(sb, bh, block_group,
+						       desc);
+				set_buffer_uptodate(bh);
+			}
+			unlock_buffer(bh);
+		}
+	} else {
+		bh = sb_bread(sb, ext4_block_bitmap(sb,desc));
+	}
	if (!bh)
		ext4_error (sb, "read_block_bitmap",
			    "Cannot read block bitmap - "
@@ -586,6 +668,7 @@
	desc->bg_free_blocks_count =
		cpu_to_le16(le16_to_cpu(desc->bg_free_blocks_count) +
			group_freed);
+	desc->bg_checksum = ext4_group_desc_csum(sbi, block_group, desc);
	spin_unlock(sb_bgl_lock(sbi, block_group));
	percpu_counter_mod(&sbi->s_freeblocks_counter, count);

@@ -1644,8 +1727,11 @@
			ret_block, goal_hits, goal_attempts);

	spin_lock(sb_bgl_lock(sbi, group_no));
+	if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))
+		gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);
	gdp->bg_free_blocks_count =
			cpu_to_le16(le16_to_cpu(gdp->bg_free_blocks_count)-num);
+	gdp->bg_checksum = ext4_group_desc_csum(sbi, group_no, gdp);
	spin_unlock(sb_bgl_lock(sbi, group_no));
	percpu_counter_mod(&sbi->s_freeblocks_counter, -num);

Index: linux-2.6.23-rc6/fs/ext4/group.h
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6.23-rc6/fs/ext4/group.h	2007-09-18 11:38:24.000000000 -0700
@@ -0,0 +1,29 @@
+/*
+ *  linux/fs/ext4/group.h
+ *
+ * Copyright (C) 2007 Cluster File Systems, Inc
+ *
+ * Author: Andreas Dilger <[email protected]>
+ */
+
+#ifndef _LINUX_EXT4_GROUP_H
+#define _LINUX_EXT4_GROUP_H
+#if defined(CONFIG_CRC16)
+#include <linux/crc16.h>
+#endif
+
+extern __le16 ext4_group_desc_csum(struct ext4_sb_info *sbi, __u32 group,
+				   struct ext4_group_desc *gdp);
+extern int ext4_group_desc_csum_verify(struct ext4_sb_info *sbi, __u32 group,
+				       struct ext4_group_desc *gdp);
+struct buffer_head *read_block_bitmap(struct super_block *sb,
+				      unsigned int block_group);
+extern unsigned ext4_init_block_bitmap(struct super_block *sb,
+				       struct buffer_head *bh, int group,
+				       struct ext4_group_desc *desc);
+#define ext4_free_blocks_after_init(sb, group, desc)			\
+		ext4_init_block_bitmap(sb, NULL, group, desc)
+extern unsigned ext4_init_inode_bitmap(struct super_block *sb,
+				       struct buffer_head *bh, int group,
+				       struct ext4_group_desc *desc);
+#endif /* _LINUX_EXT4_GROUP_H */
Index: linux-2.6.23-rc6/fs/ext4/ialloc.c
===================================================================
--- linux-2.6.23-rc6.orig/fs/ext4/ialloc.c	2007-09-10 19:50:29.000000000 -0700
+++ linux-2.6.23-rc6/fs/ext4/ialloc.c	2007-09-18 11:38:24.000000000 -0700
@@ -28,6 +28,7 @@

#include "xattr.h"
#include "acl.h"
+#include "group.h"

/*
 * ialloc.c contains the inodes allocation and deallocation routines
@@ -43,6 +44,52 @@
 * the free blocks count in the block.
 */

+/*
+ * To avoid calling the atomic setbit hundreds or thousands of times, we only
+ * need to use it within a single byte (to ensure we get endianness right).
+ * We can use memset for the rest of the bitmap as there are no other users.
+ */
+static void mark_bitmap_end(int start_bit, int end_bit, char *bitmap)
+{
+	int i;
+
+	if (start_bit >= end_bit)
+		return;
+
+	ext4_debug("mark end bits +%d through +%d used\n", start_bit, end_bit);
+	for (i = start_bit; i < ((start_bit + 7) & ~7UL); i++)
+		ext4_set_bit(i, bitmap);
+	if (i < end_bit)
+		memset(bitmap + (i >> 3), 0xff, (end_bit - i) >> 3);
+}
+
+/* Initializes an uninitialized inode bitmap */
+unsigned ext4_init_inode_bitmap(struct super_block *sb,
+				struct buffer_head *bh, int block_group,
+				struct ext4_group_desc *gdp)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+
+	J_ASSERT_BH(bh, buffer_locked(bh));
+
+	/* If checksum is bad mark all blocks and inodes use to prevent
+	 * allocation, essentially implementing a per-group read-only flag. */
+	if (!ext4_group_desc_csum_verify(sbi, block_group, gdp)) {
+		ext4_error(sb, __FUNCTION__, "Checksum bad for group %u\n",
+			   block_group);
+		gdp->bg_free_blocks_count = 0;
+		gdp->bg_free_inodes_count = 0;
+		gdp->bg_itable_unused = 0;
+		memset(bh->b_data, 0xff, sb->s_blocksize);
+		return 0;
+	}
+
+	memset(bh->b_data, 0, (EXT4_INODES_PER_GROUP(sb) + 7) / 8);
+	mark_bitmap_end(EXT4_INODES_PER_GROUP(sb), EXT4_BLOCKS_PER_GROUP(sb),
+			bh->b_data);
+
+	return EXT4_INODES_PER_GROUP(sb);
+}

/*
 * Read the inode allocation bitmap for a given block_group, reading
@@ -59,8 +106,20 @@
	desc = ext4_get_group_desc(sb, block_group, NULL);
	if (!desc)
		goto error_out;
-
-	bh = sb_bread(sb, ext4_inode_bitmap(sb, desc));
+	if (desc->bg_flags & cpu_to_le16(EXT4_BG_INODE_UNINIT)) {
+		bh = sb_getblk(sb, ext4_inode_bitmap(sb, desc));
+		if (!buffer_uptodate(bh)) {
+			lock_buffer(bh);
+			if (!buffer_uptodate(bh)) {
+				ext4_init_inode_bitmap(sb, bh, block_group,
+						       desc);
+				set_buffer_uptodate(bh);
+			}
+			unlock_buffer(bh);
+		}
+	} else {
+		bh = sb_bread(sb, ext4_inode_bitmap(sb, desc));
+	}
	if (!bh)
		ext4_error(sb, "read_inode_bitmap",
			    "Cannot read inode bitmap - "
@@ -169,6 +228,8 @@
			if (is_directory)
				gdp->bg_used_dirs_count = cpu_to_le16(
				  le16_to_cpu(gdp->bg_used_dirs_count) - 1);
+			gdp->bg_checksum = ext4_group_desc_csum(sbi,
+							block_group, gdp);
			spin_unlock(sb_bgl_lock(sbi, block_group));
			percpu_counter_inc(&sbi->s_freeinodes_counter);
			if (is_directory)
@@ -438,7 +499,7 @@
	struct ext4_sb_info *sbi;
	int err = 0;
	struct inode *ret;
-	int i;
+	int i, free = 0;

	/* Cannot create files in a deleted directory */
	if (!dir || !dir->i_nlink)
@@ -520,11 +581,13 @@
	goto out;

got:
-	ino += group * EXT4_INODES_PER_GROUP(sb) + 1;
-	if (ino < EXT4_FIRST_INO(sb) || ino > le32_to_cpu(es->s_inodes_count)) {
-		ext4_error (sb, "ext4_new_inode",
-			    "reserved inode or inode > inodes count - "
-			    "block_group = %d, inode=%lu", group, ino);
+	ino++;
+	if ((group == 0 && ino < EXT4_FIRST_INO(sb)) ||
+	    ino > EXT4_INODES_PER_GROUP(sb)) {
+		ext4_error(sb, __FUNCTION__,
+			   "reserved inode or inode > inodes count - "
+			   "block_group = %d, inode=%lu", group,
+			   ino + group * EXT4_INODES_PER_GROUP(sb));
		err = -EIO;
		goto fail;
	}
@@ -532,13 +595,64 @@
	BUFFER_TRACE(bh2, "get_write_access");
	err = ext4_journal_get_write_access(handle, bh2);
	if (err) goto fail;
+
+	/* We may have to initialize the block bitmap if it isn't already */
+	if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_GDT_CSUM) &&
+	    gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
+		struct buffer_head *block_bh = read_block_bitmap(sb, group);
+
+		BUFFER_TRACE(block_bh, "get block bitmap access");
+		err = ext4_journal_get_write_access(handle, block_bh);
+		if (err) {
+			brelse(block_bh);
+			goto fail;
+		}
+
+		free = 0;
+		spin_lock(sb_bgl_lock(sbi, group));
+		/* recheck and clear flag under lock if we still need to */
+		if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
+			gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);
+			free = ext4_free_blocks_after_init(sb, group, gdp);
+			gdp->bg_free_blocks_count = cpu_to_le16(free);
+		}
+		spin_unlock(sb_bgl_lock(sbi, group));
+
+		/* Don't need to dirty bitmap block if we didn't change it */
+		if (free) {
+			BUFFER_TRACE(block_bh, "dirty block bitmap");
+			err = ext4_journal_dirty_metadata(handle, block_bh);
+		}
+
+		brelse(block_bh);
+		if (err)
+			goto fail;
+	}
+
	spin_lock(sb_bgl_lock(sbi, group));
+	/* If we didn't allocate from within the initialized part of the inode
+	 * table then we need to initialize up to this inode. */
+	if (EXT4_HAS_RO_COMPAT_FEATURE(sb, EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {
+		if (gdp->bg_flags & cpu_to_le16(EXT4_BG_INODE_UNINIT)) {
+			gdp->bg_flags &= cpu_to_le16(~EXT4_BG_INODE_UNINIT);
+			free = EXT4_INODES_PER_GROUP(sb);
+		} else {
+			free = EXT4_INODES_PER_GROUP(sb) -
+				le16_to_cpu(gdp->bg_itable_unused);
+		}
+
+		if (ino > free)
+			gdp->bg_itable_unused =
+				cpu_to_le16(EXT4_INODES_PER_GROUP(sb) - ino);
+	}
+
	gdp->bg_free_inodes_count =
		cpu_to_le16(le16_to_cpu(gdp->bg_free_inodes_count) - 1);
	if (S_ISDIR(mode)) {
		gdp->bg_used_dirs_count =
			cpu_to_le16(le16_to_cpu(gdp->bg_used_dirs_count) + 1);
	}
+	gdp->bg_checksum = ext4_group_desc_csum(sbi, group, gdp);
	spin_unlock(sb_bgl_lock(sbi, group));
	BUFFER_TRACE(bh2, "call ext4_journal_dirty_metadata");
	err = ext4_journal_dirty_metadata(handle, bh2);
@@ -560,7 +674,7 @@
		inode->i_gid = current->fsgid;
	inode->i_mode = mode;

-	inode->i_ino = ino;
+	inode->i_ino = ino + group * EXT4_INODES_PER_GROUP(sb);
	/* This is the optimal IO size (for stat), not the fs block size */
	inode->i_blocks = 0;
	inode->i_mtime = inode->i_atime = inode->i_ctime = ei->i_crtime =
Index: linux-2.6.23-rc6/fs/ext4/resize.c
===================================================================
--- linux-2.6.23-rc6.orig/fs/ext4/resize.c	2007-09-10 19:50:29.000000000 -0700
+++ linux-2.6.23-rc6/fs/ext4/resize.c	2007-09-18 11:38:24.000000000 -0700
@@ -16,6 +16,7 @@
#include <linux/errno.h>
#include <linux/slab.h>

+#include "group.h"

#define outside(b, first, last)	((b) < (first) || (b) >= (last))
#define inside(b, first, last)	((b) >= (first) && (b) < (last))
@@ -842,6 +843,7 @@
	ext4_inode_table_set(sb, gdp, input->inode_table); /* LV FIXME */
	gdp->bg_free_blocks_count = cpu_to_le16(input->free_blocks_count);
	gdp->bg_free_inodes_count = cpu_to_le16(EXT4_INODES_PER_GROUP(sb));
+	gdp->bg_checksum = ext4_group_desc_csum(sbi, input->group, gdp);

	/*
	 * Make the new blocks and inodes valid next.  We do this before
Index: linux-2.6.23-rc6/fs/ext4/super.c
===================================================================
--- linux-2.6.23-rc6.orig/fs/ext4/super.c	2007-09-18 11:37:28.000000000 -0700
+++ linux-2.6.23-rc6/fs/ext4/super.c	2007-09-18 11:38:24.000000000 -0700
@@ -43,6 +43,7 @@
#include "xattr.h"
#include "acl.h"
#include "namei.h"
+#include "group.h"

static int ext4_load_journal(struct super_block *, struct ext4_super_block *,
			     unsigned long journal_devnum);
@@ -1247,6 +1248,93 @@
	return res;
}

+#if !defined(CONFIG_CRC16)
+/** CRC table for the CRC-16. The poly is 0x8005 (x16 + x15 + x2 + 1) */
+__u16 const crc16_table[256] = {
+	0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
+	0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
+	0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
+	0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
+	0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
+	0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
+	0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
+	0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
+	0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
+	0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
+	0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
+	0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
+	0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
+	0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
+	0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
+	0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
+	0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
+	0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
+	0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
+	0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
+	0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
+	0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
+	0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
+	0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
+	0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
+	0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
+	0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
+	0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
+	0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
+	0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
+	0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
+	0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
+};
+
+static inline __u16 crc16_byte(__u16 crc, const __u8 data)
+{
+	return (crc >> 8) ^ crc16_table[(crc ^ data) & 0xff];
+}
+
+__u16 crc16(__u16 crc, __u8 const *buffer, size_t len)
+{
+	while (len--)
+		crc = crc16_byte(crc, *buffer++);
+	return crc;
+}
+#endif
+
+__le16 ext4_group_desc_csum(struct ext4_sb_info *sbi, __u32 block_group,
+			    struct ext4_group_desc *gdp)
+{
+	__u16 crc = 0;
+
+	if (sbi->s_es->s_feature_ro_compat &
+	    cpu_to_le32(EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {
+		int offset = offsetof(struct ext4_group_desc, bg_checksum);
+		__le32 le_group = cpu_to_le32(block_group);
+
+		crc = crc16(~0, sbi->s_es->s_uuid, sizeof(sbi->s_es->s_uuid));
+		crc = crc16(crc, (__u8 *)&le_group, sizeof(le_group));
+		crc = crc16(crc, (__u8 *)gdp, offset);
+		offset += sizeof(gdp->bg_checksum); /* skip checksum */
+		/* for checksum of struct ext4_group_desc do the rest...*/
+		if ((sbi->s_es->s_feature_incompat &
+		     cpu_to_le32(EXT4_FEATURE_INCOMPAT_64BIT)) &&
+		    offset < le16_to_cpu(sbi->s_es->s_desc_size))
+			crc = crc16(crc, (__u8 *)gdp + offset,
+				    le16_to_cpu(sbi->s_es->s_desc_size) -
+					offset);
+	}
+
+	return cpu_to_le16(crc);
+}
+
+int ext4_group_desc_csum_verify(struct ext4_sb_info *sbi, __u32 block_group,
+				struct ext4_group_desc *gdp)
+{
+	if ((sbi->s_es->s_feature_ro_compat &
+	     cpu_to_le32(EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) &&
+	    (gdp->bg_checksum != ext4_group_desc_csum(sbi, block_group, gdp)))
+		return 0;
+
+	return 1;
+}
+
/* Called at mount-time, super-block is locked */
static int ext4_check_descriptors (struct super_block * sb)
{
@@ -1301,6 +1389,14 @@
				    i, inode_table);
			return 0;
		}
+		if (!ext4_group_desc_csum_verify(sbi, i, gdp)) {
+			ext4_error(sb, __FUNCTION__,
+				   "Checksum for group %d failed (%u!=%u)\n", i,
+				   le16_to_cpu(ext4_group_desc_csum(sbi, i,
+								    gdp)),
+				   le16_to_cpu(gdp->bg_checksum));
+			return 0;
+		}
		first_block += EXT4_BLOCKS_PER_GROUP(sb);
		gdp = (struct ext4_group_desc *)
			((__u8 *)gdp + EXT4_DESC_SIZE(sb));
Index: linux-2.6.23-rc6/include/linux/ext4_fs.h
===================================================================
--- linux-2.6.23-rc6.orig/include/linux/ext4_fs.h	2007-09-18 11:37:28.000000000 -0700
+++ linux-2.6.23-rc6/include/linux/ext4_fs.h	2007-09-18 11:38:24.000000000 -0700
@@ -123,19 +123,25 @@
 */
struct ext4_group_desc
{
-	__le32	bg_block_bitmap;		/* Blocks bitmap block */
-	__le32	bg_inode_bitmap;		/* Inodes bitmap block */
+	__le32	bg_block_bitmap;	/* Blocks bitmap block */
+	__le32	bg_inode_bitmap;	/* Inodes bitmap block */
	__le32	bg_inode_table;		/* Inodes table block */
	__le16	bg_free_blocks_count;	/* Free blocks count */
	__le16	bg_free_inodes_count;	/* Free inodes count */
	__le16	bg_used_dirs_count;	/* Directories count */
-	__u16	bg_flags;
-	__u32	bg_reserved[3];
+	__u16	bg_flags;		/* EXT4_BG_flags (INODE_UNINIT, etc) */
+	__u32	bg_reserved[2];		/* Likely block/inode bitmap checksum */
+	__le16  bg_itable_unused;	/* Unused inodes count */
+	__le16  bg_checksum;		/* crc16(sb_uuid+group+desc) */
	__le32	bg_block_bitmap_hi;	/* Blocks bitmap block MSB */
	__le32	bg_inode_bitmap_hi;	/* Inodes bitmap block MSB */
	__le32	bg_inode_table_hi;	/* Inodes table block MSB */
};

+#define EXT4_BG_INODE_UNINIT	0x0001 /* Inode table/bitmap not in use */
+#define EXT4_BG_BLOCK_UNINIT	0x0002 /* Block bitmap not in use */
+#define EXT4_BG_INODE_ZEROED	0x0004 /* On-disk itable initialized to zero */
+
#ifdef __KERNEL__
#include <linux/ext4_fs_i.h>
#include <linux/ext4_fs_sb.h>
@@ -693,6 +699,7 @@
#define EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER	0x0001
#define EXT4_FEATURE_RO_COMPAT_LARGE_FILE	0x0002
#define EXT4_FEATURE_RO_COMPAT_BTREE_DIR	0x0004
+#define EXT4_FEATURE_RO_COMPAT_GDT_CSUM		0x0010
#define EXT4_FEATURE_RO_COMPAT_DIR_NLINK	0x0020
#define EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE	0x0040

@@ -712,6 +719,7 @@
					 EXT4_FEATURE_INCOMPAT_64BIT)
#define EXT4_FEATURE_RO_COMPAT_SUPP	(EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER| \
					 EXT4_FEATURE_RO_COMPAT_LARGE_FILE| \
+					 EXT4_FEATURE_RO_COMPAT_GDT_CSUM| \
					 EXT4_FEATURE_RO_COMPAT_DIR_NLINK | \
					 EXT4_FEATURE_RO_COMPAT_EXTRA_ISIZE | \
					 EXT4_FEATURE_RO_COMPAT_BTREE_DIR)

Attachment: e2fsck-uninit.pdf
Description: Adobe PDF document

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