I'm reading the source code for ext2/ext3/ext4, and cannot figure
out how descriptor_loc() runs:
1600static ext3_fsblk_t descriptor_loc(struct super_block *sb, 1601 ext3_fsblk_t logic_sb_block, 1602 int nr) 1603{ 1604 struct ext3_sb_info *sbi = EXT3_SB(sb); 1605 unsigned long bg, first_meta_bg; 1606 int has_super = 0; 1607 1608 first_meta_bg = le32_to_cpu(sbi->s_es->s_first_meta_bg);
1) Who can setup the field ->s_first_meta_bg, mke2fs? And what does it mean?
What's a meta block group?
1609 1610 if (!EXT3_HAS_INCOMPAT_FEATURE(sb, EXT3_FEATURE_INCOMPAT_META_BG) || 1611 nr < first_meta_bg)
2) In which case, we can have "nr < first_meta_bg"?
And how does EXT3_FEATURE_INCOMPAT_META_BG affect the physical disk layout?
1612 return (logic_sb_block + nr + 1); 1613 bg = sbi->s_desc_per_block * nr; 1614 if (ext3_bg_has_super(sb, bg)) 1615 has_super = 1; 1616 return (has_super + ext3_group_first_block_no(sb, bg));
3) It always return the first block no of the specific bg, right?
Then its caller ext3_fill_super will read in the disk block.
It seems that ext3_fill_super just read many copies of one same block
(1st block), right?
I'm sure I'm wrong with it, but cannot find out the right way.
1617}
Any explanation are welcome.
thanks,
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