237 lines
6.5 KiB
Markdown
237 lines
6.5 KiB
Markdown
# ext4 jbd2
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https://github.com/yuoo655/ext4_rs
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https://github.com/yuoo655/jbd2_rs
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# ext4 与 ext2/ext3 区别
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| 功能 | ext2 | ext3 | ext4 |
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|-------------------------|------|------|------|
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| 日志记录 | ❌ | ✅ | ✅ |
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| 目录索引 | ❌ | ✅ | ✅ |
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| 在线扩容 | ❌ | ✅ | ✅ |
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| 在线碎片整理 | ❌ | ❌ | ✅ |
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| 校验和 | ❌ | ❌ | ✅ |
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| 是否采用extent | ❌ | ❌ | ✅ |
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| 无限数量的子目录 | ❌ | ❌ | ✅ |
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| 预分配 | ❌ | ❌ | ✅ |
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...
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# ext4支持状态
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| 操作 |支持情况|
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|--------------|------|
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| mount | ✅ |
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| open | ✅ |
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| lsdir | ✅ |
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| mkdir | ✅ |
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| read_file | ✅ |
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| read_link | ✅ |
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| create_file | ✅ |
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| write_file | ✅ |
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| link | ✅ |
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| umount | ❌ |
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| file_remove | ❌ |
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| dir_remove | ❌ |
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# jbd2支持状态
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| 操作 | 支持情况 |
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|---------------------|----------|
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| load_journal | ✅ |
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| journal_start | ✅ |
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| transaction_start | ✅ |
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| write_transaction | ✅ |
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| transaction_stop | ✅ |
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| journal_stop | ✅ |
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| recover | ✅ |
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| revoke block | ❌ |
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| checksum | ❌ |
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...
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# 独立组件
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```rust
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pub trait BlockDevice: Send + Sync + Any + Debug {
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fn read_offset(&self, offset: usize) -> Vec<u8>;
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fn write_offset(&self, offset: usize, data: &[u8]);
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}
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pub trait Jbd2: Send + Sync + Any + Debug {
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fn load_journal(&mut self);
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fn journal_start(&mut self);
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fn transaction_start(&mut self);
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fn write_transaction(&mut self, block_id: usize, block_data: Vec<u8>);
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fn transaction_stop(&mut self);
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fn journal_stop(&mut self);
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fn recover(&mut self);
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}
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```
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# 打开文件
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从挂载点开始ext4_dir_find_entry遍历目录来,对比文件名,找到目标文件，提取direntry中的inode号，这一步也就是查找文件路径到文件inode的过程。
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```rust
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fn ext4_generic_open(path){
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loop {
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ext4_dir_find_entry(path)
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if is_goal {
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file.inode = dir_search_result.dentry.inode;
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return Ok(EOK);
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}
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}
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}
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```
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# 读文件
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由于ext4默认所有文件都使用extent。extent记录了文件逻辑块号对应磁盘存储的物理块号。读取文件的过程就是寻找文件所有extent的过程。找到extent之后便可从extent中获取物理块号读出数据
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```rust
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pub struct Ext4Inode {
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...
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pub block: [u32; 15],
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...
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}
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```
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```rust
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pub fn ext4_file_read(&self, ext4_file: &mut Ext4File) -> Vec<u8> {
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...
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ext4_find_all_extent(&inode_ref, &mut extents);
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// 遍历extents向量，对每个extent，计算它的物理块号，然后调用read_block函数来读取数据块
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for extent in extents {
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let block_data = inode_ref.fs().block_device.read_offset(block_num as usize * BLOCK_SIZE);
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file_data.extend(block_data);
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}
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file_data
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}
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```
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# 创建文件
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- alloc inode
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- init inode
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- link
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分配inode
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```rust
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r = ext4_fs_alloc_inode(&mut child_inode_ref, ftype);
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```
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寻找inode位图找到第一个可用的位
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```rust
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ext4_bmap_bit_find_clr(data, 0, inodes_in_bg, &mut idx_in_bg);
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ext4_bmap_bit_set(&mut raw_data, idx_in_bg);
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```
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设置相应的inode计数
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```rust
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/* Modify filesystem counters */
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bg.set_free_inodes_count(&super_block, free_inodes);
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/* Increment used directories counter */
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if is_dir {
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used_dirs += 1;
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bg.set_used_dirs_count(&super_block, used_dirs);
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}
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/* Decrease unused inodes count */
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bg.set_itable_unused(&super_block, unused);
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/* Update superblock */
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super_block.decrease_free_inodes_count();
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```
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init inode设置inode基础信息
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```rust
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...
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inode.ext4_inode_set_mode(mode);
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inode.ext4_inode_set_links_cnt(0);
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...
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```
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init inode extent 信息
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```rust
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pub fn ext4_extent_tree_init(inode_ref: &mut Ext4Inode) {
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/* Initialize extent root header */
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let mut header = unsafe { *ext4_inode_get_extent_header(inode_ref) };
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ext4_extent_header_set_depth(&mut header, 0);
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ext4_extent_header_set_entries_count(&mut header, 0);
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ext4_extent_header_set_generation(&mut header, 0);
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ext4_extent_header_set_magic(&mut header, EXT4_EXTENT_MAGIC);
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ext4_extent_header_set_max_entries_count(&mut header, 4 as u16);
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}
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```
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再接着link inode号到文件名,目录项，父目录，首先找到父目录的目录项，再把当前文件的目录项添加到父目录项的尾部。
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```rust
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ext4_link::<Hal>(&mp,&root_inode,&mut child_inode_ref,path,name_len,false){
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ext4_dir_find_entry::<A>(&parent_inode, &path, len as u32, &mut dir_search_result);
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/* Add entry to parent directory */
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ext4_dir_add_entry::<A>(parent_inode, child_inode, path, len);
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}
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```
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# 写文件
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查找文件逻辑块对应的物理块，如果没有对应物理块则分配一个物理块。
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```rust
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pub fn ext4_file_write(&self, ext4_file: &mut Ext4File, data: &[u8], size: usize) {
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...
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let mut size = size;
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while size >= block_size {
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while iblk_idx < iblock_last {
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if iblk_idx < ifile_blocks {
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ext4_fs_append_inode_dblk(&mut inode_ref, &mut (iblk_idx as u32), &mut fblk);
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}
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iblk_idx += 1;
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...
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}
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size -= block_size;
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}
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for i in 0..fblock_count {
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...
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self.block_device
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.write_offset(offset, &data[idx..(idx + BLOCK_SIZE as usize)]);
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}
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...
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inode_ref.write_back_inode();
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}
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```
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分配物理块同样要从block bitmap中查询, 当分配完物理块后，就可以填写extent信息了。再把记录了逻辑块和物理块对应信息的extent插入extent树中。最后在相应的物理块中写入数据。
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```rust
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ext4_balloc_alloc_block()
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ext4_ext_insert_extent()
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```
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# checksum
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创建文件，写入文件都涉及对meta data的修改。所有meta data都有crc检验信息。修改元数据后，需设置校验信息，然后写入磁盘
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```rust
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例
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pub fn sync_inode_to_disk_with_csum(
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&mut self,
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block_device: Arc<dyn BlockDevice>,
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super_block: &Ext4Superblock,
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inode_id: u32,
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) -> Result<()> {
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self.set_inode_checksum(super_block, inode_id);
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self.sync_inode_to_disk(block_device, super_block, inode_id)
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}
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``` |