RT-AC56 3.0.0.4.374.37 core

[tomato.git] / release / src-rt-6.x.4708 / linux / linux-2.6.36 / fs / ext4 / balloc.c

/*
 *  linux/fs/ext4/balloc.c
 *
 * Copyright (C) 1992, 1993, 1994, 1995
 * Remy Card (card@masi.ibp.fr)
 * Laboratoire MASI - Institut Blaise Pascal
 * Universite Pierre et Marie Curie (Paris VI)
 *
 *  Enhanced block allocation by Stephen Tweedie (sct@redhat.com), 1993
 *  Big-endian to little-endian byte-swapping/bitmaps by
 *        David S. Miller (davem@caip.rutgers.edu), 1995
 */

#include <linux/time.h>
#include <linux/capability.h>
#include <linux/fs.h>
#include <linux/jbd2.h>
#include <linux/quotaops.h>
#include <linux/buffer_head.h>
#include "ext4.h"
#include "ext4_jbd2.h"
#include "mballoc.h"

/*
 * balloc.c contains the blocks allocation and deallocation routines
 */

/*
 * Calculate the block group number and offset, given a block number
 */
void ext4_get_group_no_and_offset(struct super_block *sb, ext4_fsblk_t blocknr,
                ext4_group_t *blockgrpp, ext4_grpblk_t *offsetp)
{
        struct ext4_super_block *es = EXT4_SB(sb)->s_es;
        ext4_grpblk_t offset;

        blocknr = blocknr - le32_to_cpu(es->s_first_data_block);
        offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb));
        if (offsetp)
                *offsetp = offset;
        if (blockgrpp)
                *blockgrpp = blocknr;

}

static int ext4_block_in_group(struct super_block *sb, ext4_fsblk_t block,
                        ext4_group_t block_group)
{
        ext4_group_t actual_group;
        ext4_get_group_no_and_offset(sb, block, &actual_group, NULL);
        if (actual_group == block_group)
                return 1;
        return 0;
}

static int ext4_group_used_meta_blocks(struct super_block *sb,
                                       ext4_group_t block_group,
                                       struct ext4_group_desc *gdp)
{
        ext4_fsblk_t tmp;
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        /* block bitmap, inode bitmap, and inode table blocks */
        int used_blocks = sbi->s_itb_per_group + 2;

        if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG)) {
                if (!ext4_block_in_group(sb, ext4_block_bitmap(sb, gdp),
                                        block_group))
                        used_blocks--;

                if (!ext4_block_in_group(sb, ext4_inode_bitmap(sb, gdp),
                                        block_group))
                        used_blocks--;

                tmp = ext4_inode_table(sb, gdp);
                for (; tmp < ext4_inode_table(sb, gdp) +
                                sbi->s_itb_per_group; tmp++) {
                        if (!ext4_block_in_group(sb, tmp, block_group))
                                used_blocks -= 1;
                }
        }
        return used_blocks;
}

/* 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,
                 ext4_group_t block_group, struct ext4_group_desc *gdp)
{
        int bit, bit_max;
        ext4_group_t ngroups = ext4_get_groups_count(sb);
        unsigned free_blocks, group_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, "Checksum bad for group %u",
                                        block_group);
                        ext4_free_blks_set(sb, gdp, 0);
                        ext4_free_inodes_set(sb, gdp, 0);
                        ext4_itable_unused_set(sb, gdp, 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 */
                bit_max += ext4_bg_num_gdb(sb, block_group);
        }

        if (block_group == ngroups - 1) {
                /*
                 * Even though mke2fs always initialize first and last group
                 * if some other tool enabled the EXT4_BG_BLOCK_UNINIT we need
                 * to make sure we calculate the right free blocks
                 */
                group_blocks = ext4_blocks_count(sbi->s_es) -
                        ext4_group_first_block_no(sb, ngroups - 1);
        } else {
                group_blocks = EXT4_BLOCKS_PER_GROUP(sb);
        }

        free_blocks = group_blocks - bit_max;

        if (bh) {
                ext4_fsblk_t start, tmp;
                int flex_bg = 0;

                for (bit = 0; bit < bit_max; bit++)
                        ext4_set_bit(bit, bh->b_data);

                start = ext4_group_first_block_no(sb, block_group);

                if (EXT4_HAS_INCOMPAT_FEATURE(sb,
                                              EXT4_FEATURE_INCOMPAT_FLEX_BG))
                        flex_bg = 1;

                /* Set bits for block and inode bitmaps, and inode table */
                tmp = ext4_block_bitmap(sb, gdp);
                if (!flex_bg || ext4_block_in_group(sb, tmp, block_group))
                        ext4_set_bit(tmp - start, bh->b_data);

                tmp = ext4_inode_bitmap(sb, gdp);
                if (!flex_bg || ext4_block_in_group(sb, tmp, block_group))
                        ext4_set_bit(tmp - start, bh->b_data);

                tmp = ext4_inode_table(sb, gdp);
                for (; tmp < ext4_inode_table(sb, gdp) +
                                sbi->s_itb_per_group; tmp++) {
                        if (!flex_bg ||
                                ext4_block_in_group(sb, tmp, block_group))
                                ext4_set_bit(tmp - start, bh->b_data);
                }
                /*
                 * Also if the number of blocks within the group is
                 * less than the blocksize * 8 ( which is the size
                 * of bitmap ), set rest of the block bitmap to 1
                 */
                mark_bitmap_end(group_blocks, sb->s_blocksize * 8, bh->b_data);
        }
        return free_blocks - ext4_group_used_meta_blocks(sb, block_group, gdp);
}


/*
 * The free blocks are managed by bitmaps.  A file system contains several
 * blocks groups.  Each group contains 1 bitmap block for blocks, 1 bitmap
 * block for inodes, N blocks for the inode table and data blocks.
 *
 * The file system contains group descriptors which are located after the
 * super block.  Each descriptor contains the number of the bitmap block and
 * the free blocks count in the block.  The descriptors are loaded in memory
 * when a file system is mounted (see ext4_fill_super).
 */

/**
 * ext4_get_group_desc() -- load group descriptor from disk
 * @sb:                 super block
 * @block_group:        given block group
 * @bh:                 pointer to the buffer head to store the block
 *                      group descriptor
 */
struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
                                             ext4_group_t block_group,
                                             struct buffer_head **bh)
{
        unsigned int group_desc;
        unsigned int offset;
        ext4_group_t ngroups = ext4_get_groups_count(sb);
        struct ext4_group_desc *desc;
        struct ext4_sb_info *sbi = EXT4_SB(sb);

        if (block_group >= ngroups) {
                ext4_error(sb, "block_group >= groups_count - block_group = %u,"
                           " groups_count = %u", block_group, ngroups);

                return NULL;
        }

        group_desc = block_group >> EXT4_DESC_PER_BLOCK_BITS(sb);
        offset = block_group & (EXT4_DESC_PER_BLOCK(sb) - 1);
        if (!sbi->s_group_desc[group_desc]) {
                ext4_error(sb, "Group descriptor not loaded - "
                           "block_group = %u, group_desc = %u, desc = %u",
                           block_group, group_desc, offset);
                return NULL;
        }

        desc = (struct ext4_group_desc *)(
                (__u8 *)sbi->s_group_desc[group_desc]->b_data +
                offset * EXT4_DESC_SIZE(sb));
        if (bh)
                *bh = sbi->s_group_desc[group_desc];
        return desc;
}

static int ext4_valid_block_bitmap(struct super_block *sb,
                                        struct ext4_group_desc *desc,
                                        unsigned int block_group,
                                        struct buffer_head *bh)
{
        ext4_grpblk_t offset;
        ext4_grpblk_t next_zero_bit;
        ext4_fsblk_t bitmap_blk;
        ext4_fsblk_t group_first_block;

        if (EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_FLEX_BG)) {
                /* with FLEX_BG, the inode/block bitmaps and itable
                 * blocks may not be in the group at all
                 * so the bitmap validation will be skipped for those groups
                 * or it has to also read the block group where the bitmaps
                 * are located to verify they are set.
                 */
                return 1;
        }
        group_first_block = ext4_group_first_block_no(sb, block_group);

        /* check whether block bitmap block number is set */
        bitmap_blk = ext4_block_bitmap(sb, desc);
        offset = bitmap_blk - group_first_block;
        if (!ext4_test_bit(offset, bh->b_data))
                /* bad block bitmap */
                goto err_out;

        /* check whether the inode bitmap block number is set */
        bitmap_blk = ext4_inode_bitmap(sb, desc);
        offset = bitmap_blk - group_first_block;
        if (!ext4_test_bit(offset, bh->b_data))
                /* bad block bitmap */
                goto err_out;

        /* check whether the inode table block number is set */
        bitmap_blk = ext4_inode_table(sb, desc);
        offset = bitmap_blk - group_first_block;
        next_zero_bit = ext4_find_next_zero_bit(bh->b_data,
                                offset + EXT4_SB(sb)->s_itb_per_group,
                                offset);
        if (next_zero_bit >= offset + EXT4_SB(sb)->s_itb_per_group)
                /* good bitmap for inode tables */
                return 1;

err_out:
        ext4_error(sb, "Invalid block bitmap - block_group = %d, block = %llu",
                        block_group, bitmap_blk);
        return 0;
}
/**
 * ext4_read_block_bitmap()
 * @sb:                 super block
 * @block_group:        given block group
 *
 * Read the bitmap for a given block_group,and validate the
 * bits for block/inode/inode tables are set in the bitmaps
 *
 * Return buffer_head on success or NULL in case of failure.
 */
struct buffer_head *
ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
{
        struct ext4_group_desc *desc;
        struct buffer_head *bh = NULL;
        ext4_fsblk_t bitmap_blk;

        desc = ext4_get_group_desc(sb, block_group, NULL);
        if (!desc)
                return NULL;
        bitmap_blk = ext4_block_bitmap(sb, desc);
        bh = sb_getblk(sb, bitmap_blk);
        if (unlikely(!bh)) {
                ext4_error(sb, "Cannot read block bitmap - "
                            "block_group = %u, block_bitmap = %llu",
                            block_group, bitmap_blk);
                return NULL;
        }

        if (bitmap_uptodate(bh))
                return bh;

        lock_buffer(bh);
        if (bitmap_uptodate(bh)) {
                unlock_buffer(bh);
                return bh;
        }
        ext4_lock_group(sb, block_group);
        if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
                ext4_init_block_bitmap(sb, bh, block_group, desc);
                set_bitmap_uptodate(bh);
                set_buffer_uptodate(bh);
                ext4_unlock_group(sb, block_group);
                unlock_buffer(bh);
                return bh;
        }
        ext4_unlock_group(sb, block_group);
        if (buffer_uptodate(bh)) {
                /*
                 * if not uninit if bh is uptodate,
                 * bitmap is also uptodate
                 */
                set_bitmap_uptodate(bh);
                unlock_buffer(bh);
                return bh;
        }
        /*
         * submit the buffer_head for read. We can
         * safely mark the bitmap as uptodate now.
         * We do it here so the bitmap uptodate bit
         * get set with buffer lock held.
         */
        set_bitmap_uptodate(bh);
        if (bh_submit_read(bh) < 0) {
                put_bh(bh);
                ext4_error(sb, "Cannot read block bitmap - "
                            "block_group = %u, block_bitmap = %llu",
                            block_group, bitmap_blk);
                return NULL;
        }
        ext4_valid_block_bitmap(sb, desc, block_group, bh);
        /*
         * file system mounted not to panic on error,
         * continue with corrupt bitmap
         */
        return bh;
}

/**
 * ext4_add_groupblocks() -- Add given blocks to an existing group
 * @handle:                     handle to this transaction
 * @sb:                         super block
 * @block:                      start physcial block to add to the block group
 * @count:                      number of blocks to free
 *
 * This marks the blocks as free in the bitmap. We ask the
 * mballoc to reload the buddy after this by setting group
 * EXT4_GROUP_INFO_NEED_INIT_BIT flag
 */
void ext4_add_groupblocks(handle_t *handle, struct super_block *sb,
                         ext4_fsblk_t block, unsigned long count)
{
        struct buffer_head *bitmap_bh = NULL;
        struct buffer_head *gd_bh;
        ext4_group_t block_group;
        ext4_grpblk_t bit;
        unsigned int i;
        struct ext4_group_desc *desc;
        struct ext4_sb_info *sbi = EXT4_SB(sb);
        int err = 0, ret, blk_free_count;
        ext4_grpblk_t blocks_freed;
        struct ext4_group_info *grp;

        ext4_debug("Adding block(s) %llu-%llu\n", block, block + count - 1);

        ext4_get_group_no_and_offset(sb, block, &block_group, &bit);
        grp = ext4_get_group_info(sb, block_group);
        /*
         * Check to see if we are freeing blocks across a group
         * boundary.
         */
        if (bit + count > EXT4_BLOCKS_PER_GROUP(sb)) {
                goto error_return;
        }
        bitmap_bh = ext4_read_block_bitmap(sb, block_group);
        if (!bitmap_bh)
                goto error_return;
        desc = ext4_get_group_desc(sb, block_group, &gd_bh);
        if (!desc)
                goto error_return;

        if (in_range(ext4_block_bitmap(sb, desc), block, count) ||
            in_range(ext4_inode_bitmap(sb, desc), block, count) ||
            in_range(block, ext4_inode_table(sb, desc), sbi->s_itb_per_group) ||
            in_range(block + count - 1, ext4_inode_table(sb, desc),
                     sbi->s_itb_per_group)) {
                ext4_error(sb, "Adding blocks in system zones - "
                           "Block = %llu, count = %lu",
                           block, count);
                goto error_return;
        }

        /*
         * We are about to add blocks to the bitmap,
         * so we need undo access.
         */
        BUFFER_TRACE(bitmap_bh, "getting undo access");
        err = ext4_journal_get_undo_access(handle, bitmap_bh);
        if (err)
                goto error_return;

        /*
         * We are about to modify some metadata.  Call the journal APIs
         * to unshare ->b_data if a currently-committing transaction is
         * using it
         */
        BUFFER_TRACE(gd_bh, "get_write_access");
        err = ext4_journal_get_write_access(handle, gd_bh);
        if (err)
                goto error_return;
        /*
         * make sure we don't allow a parallel init on other groups in the
         * same buddy cache
         */
        down_write(&grp->alloc_sem);
        for (i = 0, blocks_freed = 0; i < count; i++) {
                BUFFER_TRACE(bitmap_bh, "clear bit");
                if (!ext4_clear_bit_atomic(ext4_group_lock_ptr(sb, block_group),
                                                bit + i, bitmap_bh->b_data)) {
                        ext4_error(sb, "bit already cleared for block %llu",
                                   (ext4_fsblk_t)(block + i));
                        BUFFER_TRACE(bitmap_bh, "bit already cleared");
                } else {
                        blocks_freed++;
                }
        }
        ext4_lock_group(sb, block_group);
        blk_free_count = blocks_freed + ext4_free_blks_count(sb, desc);
        ext4_free_blks_set(sb, desc, blk_free_count);
        desc->bg_checksum = ext4_group_desc_csum(sbi, block_group, desc);
        ext4_unlock_group(sb, block_group);
        percpu_counter_add(&sbi->s_freeblocks_counter, blocks_freed);

        if (sbi->s_log_groups_per_flex) {
                ext4_group_t flex_group = ext4_flex_group(sbi, block_group);
                atomic_add(blocks_freed,
                           &sbi->s_flex_groups[flex_group].free_blocks);
        }
        /*
         * request to reload the buddy with the
         * new bitmap information
         */
        set_bit(EXT4_GROUP_INFO_NEED_INIT_BIT, &(grp->bb_state));
        grp->bb_free += blocks_freed;
        up_write(&grp->alloc_sem);

        /* We dirtied the bitmap block */
        BUFFER_TRACE(bitmap_bh, "dirtied bitmap block");
        err = ext4_handle_dirty_metadata(handle, NULL, bitmap_bh);

        /* And the group descriptor block */
        BUFFER_TRACE(gd_bh, "dirtied group descriptor block");
        ret = ext4_handle_dirty_metadata(handle, NULL, gd_bh);
        if (!err)
                err = ret;

error_return:
        brelse(bitmap_bh);
        ext4_std_error(sb, err);
        return;
}

/**
 * ext4_has_free_blocks()
 * @sbi:        in-core super block structure.
 * @nblocks:    number of needed blocks
 *
 * Check if filesystem has nblocks free & available for allocation.
 * On success return 1, return 0 on failure.
 */
int ext4_has_free_blocks(struct ext4_sb_info *sbi, s64 nblocks)
{
        s64 free_blocks, dirty_blocks, root_blocks;
        struct percpu_counter *fbc = &sbi->s_freeblocks_counter;
        struct percpu_counter *dbc = &sbi->s_dirtyblocks_counter;

        free_blocks  = percpu_counter_read_positive(fbc);
        dirty_blocks = percpu_counter_read_positive(dbc);
        root_blocks = ext4_r_blocks_count(sbi->s_es);

        if (free_blocks - (nblocks + root_blocks + dirty_blocks) <
                                                EXT4_FREEBLOCKS_WATERMARK) {
                free_blocks  = percpu_counter_sum_positive(fbc);
                dirty_blocks = percpu_counter_sum_positive(dbc);
                if (dirty_blocks < 0) {
                        printk(KERN_CRIT "Dirty block accounting "
                                        "went wrong %lld\n",
                                        (long long)dirty_blocks);
                }
        }
        /* Check whether we have space after
         * accounting for current dirty blocks & root reserved blocks.
         */
        if (free_blocks >= ((root_blocks + nblocks) + dirty_blocks))
                return 1;

        /* Hm, nope.  Are (enough) root reserved blocks available? */
        if (sbi->s_resuid == current_fsuid() ||
            ((sbi->s_resgid != 0) && in_group_p(sbi->s_resgid)) ||
            capable(CAP_SYS_RESOURCE)) {
                if (free_blocks >= (nblocks + dirty_blocks))
                        return 1;
        }

        return 0;
}

int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
                                                s64 nblocks)
{
        if (ext4_has_free_blocks(sbi, nblocks)) {
                percpu_counter_add(&sbi->s_dirtyblocks_counter, nblocks);
                return 0;
        } else
                return -ENOSPC;
}

/**
 * ext4_should_retry_alloc()
 * @sb:                 super block
 * @retries             number of attemps has been made
 *
 * ext4_should_retry_alloc() is called when ENOSPC is returned, and if
 * it is profitable to retry the operation, this function will wait
 * for the current or commiting transaction to complete, and then
 * return TRUE.
 *
 * if the total number of retries exceed three times, return FALSE.
 */
int ext4_should_retry_alloc(struct super_block *sb, int *retries)
{
        if (!ext4_has_free_blocks(EXT4_SB(sb), 1) ||
            (*retries)++ > 3 ||
            !EXT4_SB(sb)->s_journal)
                return 0;

        jbd_debug(1, "%s: retrying operation after ENOSPC\n", sb->s_id);

        return jbd2_journal_force_commit_nested(EXT4_SB(sb)->s_journal);
}

/*
 * ext4_new_meta_blocks() -- allocate block for meta data (indexing) blocks
 *
 * @handle:             handle to this transaction
 * @inode:              file inode
 * @goal:               given target block(filesystem wide)
 * @count:              pointer to total number of blocks needed
 * @errp:               error code
 *
 * Return 1st allocated block number on success, *count stores total account
 * error stores in errp pointer
 */
ext4_fsblk_t ext4_new_meta_blocks(handle_t *handle, struct inode *inode,
                ext4_fsblk_t goal, unsigned long *count, int *errp)
{
        struct ext4_allocation_request ar;
        ext4_fsblk_t ret;

        memset(&ar, 0, sizeof(ar));
        /* Fill with neighbour allocated blocks */
        ar.inode = inode;
        ar.goal = goal;
        ar.len = count ? *count : 1;

        ret = ext4_mb_new_blocks(handle, &ar, errp);
        if (count)
                *count = ar.len;
        /*
         * Account for the allocated meta blocks.  We will never
         * fail EDQUOT for metdata, but we do account for it.
         */
        if (!(*errp) && EXT4_I(inode)->i_delalloc_reserved_flag) {
                spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
                EXT4_I(inode)->i_allocated_meta_blocks += ar.len;
                spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
                dquot_alloc_block_nofail(inode, ar.len);
        }
        return ret;
}

/**
 * ext4_count_free_blocks() -- count filesystem free blocks
 * @sb:         superblock
 *
 * Adds up the number of free blocks from each block group.
 */
ext4_fsblk_t ext4_count_free_blocks(struct super_block *sb)
{
        ext4_fsblk_t desc_count;
        struct ext4_group_desc *gdp;
        ext4_group_t i;
        ext4_group_t ngroups = ext4_get_groups_count(sb);
#ifdef EXT4FS_DEBUG
        struct ext4_super_block *es;
        ext4_fsblk_t bitmap_count;
        unsigned int x;
        struct buffer_head *bitmap_bh = NULL;

        es = EXT4_SB(sb)->s_es;
        desc_count = 0;
        bitmap_count = 0;
        gdp = NULL;

        for (i = 0; i < ngroups; i++) {
                gdp = ext4_get_group_desc(sb, i, NULL);
                if (!gdp)
                        continue;
                desc_count += ext4_free_blks_count(sb, gdp);
                brelse(bitmap_bh);
                bitmap_bh = ext4_read_block_bitmap(sb, i);
                if (bitmap_bh == NULL)
                        continue;

                x = ext4_count_free(bitmap_bh, sb->s_blocksize);
                printk(KERN_DEBUG "group %u: stored = %d, counted = %u\n",
                        i, ext4_free_blks_count(sb, gdp), x);
                bitmap_count += x;
        }
        brelse(bitmap_bh);
        printk(KERN_DEBUG "ext4_count_free_blocks: stored = %llu"
                ", computed = %llu, %llu\n", ext4_free_blocks_count(es),
               desc_count, bitmap_count);
        return bitmap_count;
#else
        desc_count = 0;
        for (i = 0; i < ngroups; i++) {
                gdp = ext4_get_group_desc(sb, i, NULL);
                if (!gdp)
                        continue;
                desc_count += ext4_free_blks_count(sb, gdp);
        }

        return desc_count;
#endif
}

static inline int test_root(ext4_group_t a, int b)
{
        int num = b;

        while (a > num)
                num *= b;
        return num == a;
}

static int ext4_group_sparse(ext4_group_t group)
{
        if (group <= 1)
                return 1;
        if (!(group & 1))
                return 0;
        return (test_root(group, 7) || test_root(group, 5) ||
                test_root(group, 3));
}

/**
 *      ext4_bg_has_super - number of blocks used by the superblock in group
 *      @sb: superblock for filesystem
 *      @group: group number to check
 *
 *      Return the number of blocks used by the superblock (primary or backup)
 *      in this group.  Currently this will be only 0 or 1.
 */
int ext4_bg_has_super(struct super_block *sb, ext4_group_t group)
{
        if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
                                EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER) &&
                        !ext4_group_sparse(group))
                return 0;
        return 1;
}

static unsigned long ext4_bg_num_gdb_meta(struct super_block *sb,
                                        ext4_group_t group)
{
        unsigned long metagroup = group / EXT4_DESC_PER_BLOCK(sb);
        ext4_group_t first = metagroup * EXT4_DESC_PER_BLOCK(sb);
        ext4_group_t last = first + EXT4_DESC_PER_BLOCK(sb) - 1;

        if (group == first || group == first + 1 || group == last)
                return 1;
        return 0;
}

static unsigned long ext4_bg_num_gdb_nometa(struct super_block *sb,
                                        ext4_group_t group)
{
        if (!ext4_bg_has_super(sb, group))
                return 0;

        if (EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG))
                return le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
        else
                return EXT4_SB(sb)->s_gdb_count;
}

/**
 *      ext4_bg_num_gdb - number of blocks used by the group table in group
 *      @sb: superblock for filesystem
 *      @group: group number to check
 *
 *      Return the number of blocks used by the group descriptor table
 *      (primary or backup) in this group.  In the future there may be a
 *      different number of descriptor blocks in each group.
 */
unsigned long ext4_bg_num_gdb(struct super_block *sb, ext4_group_t group)
{
        unsigned long first_meta_bg =
                        le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
        unsigned long metagroup = group / EXT4_DESC_PER_BLOCK(sb);

        if (!EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG) ||
                        metagroup < first_meta_bg)
                return ext4_bg_num_gdb_nometa(sb, group);

        return ext4_bg_num_gdb_meta(sb,group);

}