Hi, I'm reading the source code for ext2/ext3/ext4, and cannot figure out how descriptor_loc() runs: 1600 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1600>static ext3_fsblk_t <http://lxr.linux.no/linux+*/+code=ext3_fsblk_t> descriptor_loc <http://lxr.linux.no/linux+*/+code=descriptor_loc>(struct super_block <http://lxr.linux.no/linux+*/+code=super_block> *sb <http://lxr.linux.no/linux+*/+code=sb>,1601 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1601> ext3_fsblk_t <http://lxr.linux.no/linux+*/+code=ext3_fsblk_t> logic_sb_block <http://lxr.linux.no/linux+*/+code=logic_sb_block>,1602 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1602> int nr <http://lxr.linux.no/linux+*/+code=nr>)1603 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1603>{1604 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1604> struct ext3_sb_info <http://lxr.linux.no/linux+*/+code=ext3_sb_info> *sbi <http://lxr.linux.no/linux+*/+code=sbi> = EXT3_SB <http://lxr.linux.no/linux+*/+code=EXT3_SB>(sb <http://lxr.linux.no/linux+*/+code=sb>);1605 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1605> unsigned long bg <http://lxr.linux.no/linux+*/+code=bg>, first_meta_bg <http://lxr.linux.no/linux+*/+code=first_meta_bg>;1606 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1606> int has_super <http://lxr.linux.no/linux+*/+code=has_super> = 0;1607 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1607>1608 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1608> first_meta_bg <http://lxr.linux.no/linux+*/+code=first_meta_bg> = le32_to_cpu <http://lxr.linux.no/linux+*/+code=le32_to_cpu>(sbi <http://lxr.linux.no/linux+*/+code=sbi>->s_es <http://lxr.linux.no/linux+*/+code=s_es>->s_first_meta_bg <http://lxr.linux.no/linux+*/+code=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 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1609>1610 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1610> if (!EXT3_HAS_INCOMPAT_FEATURE <http://lxr.linux.no/linux+*/+code=EXT3_HAS_INCOMPAT_FEATURE>(sb <http://lxr.linux.no/linux+*/+code=sb>, EXT3_FEATURE_INCOMPAT_META_BG <http://lxr.linux.no/linux+*/+code=EXT3_FEATURE_INCOMPAT_META_BG>) ||1611 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1611> nr <http://lxr.linux.no/linux+*/+code=nr> < first_meta_bg <http://lxr.linux.no/linux+*/+code=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 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1612> return (logic_sb_block <http://lxr.linux.no/linux+*/+code=logic_sb_block> + nr <http://lxr.linux.no/linux+*/+code=nr> + 1);1613 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1613> bg <http://lxr.linux.no/linux+*/+code=bg> = sbi <http://lxr.linux.no/linux+*/+code=sbi>->s_desc_per_block <http://lxr.linux.no/linux+*/+code=s_desc_per_block> * nr <http://lxr.linux.no/linux+*/+code=nr>;1614 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1614> if (ext3_bg_has_super <http://lxr.linux.no/linux+*/+code=ext3_bg_has_super>(sb <http://lxr.linux.no/linux+*/+code=sb>, bg <http://lxr.linux.no/linux+*/+code=bg>))1615 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1615> has_super <http://lxr.linux.no/linux+*/+code=has_super> = 1;1616 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1616> return (has_super <http://lxr.linux.no/linux+*/+code=has_super> + ext3_group_first_block_no <http://lxr.linux.no/linux+*/+code=ext3_group_first_block_no>(sb <http://lxr.linux.no/linux+*/+code=sb>, bg <http://lxr.linux.no/linux+*/+code=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 <http://lxr.linux.no/linux+*/fs/ext3/super.c#L1617>} Any explanation are welcome. thanks,