+/*
+ * Note: calculating the overhead so we can be compatible with
+ * historical BSD practice is quite difficult in the face of
+ * clusters/bigalloc. This is because multiple metadata blocks from
+ * different block group can end up in the same allocation cluster.
+ * Calculating the exact overhead in the face of clustered allocation
+ * requires either O(all block bitmaps) in memory or O(number of block
+ * groups**2) in time. We will still calculate the superblock for
+ * older file systems --- and if we come across with a bigalloc file
+ * system with zero in s_overhead_clusters the estimate will be close to
+ * correct especially for very large cluster sizes --- but for newer
+ * file systems, it's better to calculate this figure once at mkfs
+ * time, and store it in the superblock. If the superblock value is
+ * present (even for non-bigalloc file systems), we will use it.
+ */
+static int count_overhead(struct super_block *sb, ext4_group_t grp,
+ char *buf)
+{
+ struct ext4_sb_info *sbi = EXT4_SB(sb);
+ struct ext4_group_desc *gdp;
+ ext4_fsblk_t first_block, last_block, b;
+ ext4_group_t i, ngroups = ext4_get_groups_count(sb);
+ int s, j, count = 0;
+
+ first_block = le32_to_cpu(sbi->s_es->s_first_data_block) +
+ (grp * EXT4_BLOCKS_PER_GROUP(sb));
+ last_block = first_block + EXT4_BLOCKS_PER_GROUP(sb) - 1;
+ for (i = 0; i < ngroups; i++) {
+ gdp = ext4_get_group_desc(sb, i, NULL);
+ b = ext4_block_bitmap(sb, gdp);
+ if (b >= first_block && b <= last_block) {
+ ext4_set_bit(EXT4_B2C(sbi, b - first_block), buf);
+ count++;
+ }
+ b = ext4_inode_bitmap(sb, gdp);
+ if (b >= first_block && b <= last_block) {
+ ext4_set_bit(EXT4_B2C(sbi, b - first_block), buf);
+ count++;
+ }
+ b = ext4_inode_table(sb, gdp);
+ if (b >= first_block && b + sbi->s_itb_per_group <= last_block)
+ for (j = 0; j < sbi->s_itb_per_group; j++, b++) {
+ int c = EXT4_B2C(sbi, b - first_block);
+ ext4_set_bit(c, buf);
+ count++;
+ }
+ if (i != grp)
+ continue;
+ s = 0;
+ if (ext4_bg_has_super(sb, grp)) {
+ ext4_set_bit(s++, buf);
+ count++;
+ }
+ for (j = ext4_bg_num_gdb(sb, grp); j > 0; j--) {
+ ext4_set_bit(EXT4_B2C(sbi, s++), buf);
+ count++;
+ }
+ }
+ if (!count)
+ return 0;
+ return EXT4_CLUSTERS_PER_GROUP(sb) -
+ ext4_count_free(buf, EXT4_CLUSTERS_PER_GROUP(sb) / 8);
+}
+
+/*
+ * Compute the overhead and stash it in sbi->s_overhead
+ */
+int ext4_calculate_overhead(struct super_block *sb)
+{
+ struct ext4_sb_info *sbi = EXT4_SB(sb);
+ struct ext4_super_block *es = sbi->s_es;
+ ext4_group_t i, ngroups = ext4_get_groups_count(sb);
+ ext4_fsblk_t overhead = 0;
+ char *buf = (char *) get_zeroed_page(GFP_KERNEL);
+
+ memset(buf, 0, PAGE_SIZE);
+ if (!buf)
+ return -ENOMEM;
+
+ /*
+ * Compute the overhead (FS structures). This is constant
+ * for a given filesystem unless the number of block groups
+ * changes so we cache the previous value until it does.
+ */
+
+ /*
+ * All of the blocks before first_data_block are overhead
+ */
+ overhead = EXT4_B2C(sbi, le32_to_cpu(es->s_first_data_block));
+
+ /*
+ * Add the overhead found in each block group
+ */
+ for (i = 0; i < ngroups; i++) {
+ int blks;
+
+ blks = count_overhead(sb, i, buf);
+ overhead += blks;
+ if (blks)
+ memset(buf, 0, PAGE_SIZE);
+ cond_resched();
+ }
+ sbi->s_overhead = overhead;
+ smp_wmb();
+ free_page((unsigned long) buf);
+ return 0;
+}
+