EROFS 源码阅读

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#erofs in-tree 驱动

in-tree 的代码也是模块的形式，以 module_init 开始

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module_init(erofs_module_init);
module_exit(erofs_module_exit);

// in erofs_module_init()
err = register_filesystem(&erofs_fs_type);

static struct file_system_type erofs_fs_type = {
	.owner          = THIS_MODULE,
	.name           = "erofs",
	.init_fs_context = erofs_init_fs_context, // init ->ops and ->fs_private
	.kill_sb        = erofs_kill_sb, // shutdown a fs
	.fs_flags       = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
};
MODULE_ALIAS_FS("erofs");

Kconfig 中包含的一些选项：

  • EROFS_FS_DEBUG DEBUG支持
  • EROFS_FS_XATTR 拓展属性支持
  • EROFS_FS_POSIX_ACL ACL支持
  • EROFS_FS_SECURITY 安全标签支持
  • EROFS_FS_ZIP 压缩支持，默认只支持读取 LZ4 压缩的数据
    • EROFS_FS_ZIP_LZMA 支持读取 microLZMA 压缩的数据
    • EROFS_FS_ZIP_DEFLATE 支持读取 DEFLATE 压缩的数据
    • EROFS_FS_PCPU_KTHREAD 用per-cpu的线程池去作异步数据解压缩
      • EROFS_FS_PCPU_KTHREAD_HIPRI 把这些线程设为高优先级
  • EROFS_FS_ONDEMAND 利用 fscache 实现 on-demand 读

#初始化流程

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static const struct fs_context_operations erofs_context_ops = {
	.parse_param	= erofs_fc_parse_param, // add a fs param
	.get_tree       = erofs_fc_get_tree, // ~mount a fs
	.reconfigure    = erofs_fc_reconfigure, // sb reconfiguration
	.free		= erofs_fc_free, // clean up
};

#内核的 fs_context 结构

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struct fs_context {
	const struct fs_context_operations *ops;
	struct file_system_type *fs_type;
	void			*fs_private;
	struct dentry		*root;  // the root of fs tree
	struct user_namespace	*user_ns;
	struct net		*net_ns;
	const struct cred	*cred;
	char			*source;  // /dev/sda1 or host:/path, etc.
	char			*subtype;
	void			*security;
	void			*s_fs_info;  // user data -> erofs_sb_info
	unsigned int		sb_flags;
	unsigned int		sb_flags_mask;
	unsigned int		s_iflags;
	enum fs_context_purpose	purpose:8;
	...
};

#EROFS 的元数据 erofs_sb_info

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struct erofs_sb_info {
	struct erofs_mount_opts opt;	/* options */
	
#ifdef CONFIG_EROFS_FS_ZIP
	/* list for all registered superblocks, mainly for shrinker */
	struct list_head list;
	struct mutex umount_mutex;

	/* managed XArray arranged in physical block number */
	struct xarray managed_pslots;

	unsigned int shrinker_run_no;
	u16 available_compr_algs;

	/* pseudo inode to manage cached pages */
	struct inode *managed_cache;

	struct erofs_sb_lz4_info lz4;
#endif	/* CONFIG_EROFS_FS_ZIP */
	struct inode *packed_inode;
	struct erofs_dev_context *devs;
	struct dax_device *dax_dev;
	u64 dax_part_off;
	u64 total_blocks;
	u32 primarydevice_blocks;

	u32 meta_blkaddr;
#ifdef CONFIG_EROFS_FS_XATTR
	u32 xattr_blkaddr;
	u32 xattr_prefix_start;
	u8 xattr_prefix_count;
	struct erofs_xattr_prefix_item *xattr_prefixes;
	unsigned int xattr_filter_reserved;
#endif
	u16 device_id_mask;	/* valid bits of device id to be used */

	unsigned char islotbits;	/* inode slot unit size in bit shift */
	unsigned char blkszbits;	/* filesystem block size in bit shift */

	u32 sb_size;			/* total superblock size */
	u32 build_time_nsec;
	u64 build_time;

	/* what we really care is nid, rather than ino.. */
	erofs_nid_t root_nid;
	erofs_nid_t packed_nid;
	/* used for statfs, f_files - f_favail */
	u64 inos;

	u8 uuid[16];                    /* 128-bit uuid for volume */
	u8 volume_name[16];             /* volume name */
	u32 feature_compat;
	u32 feature_incompat;

	/* sysfs support */
	struct kobject s_kobj;		/* /sys/fs/erofs/<devname> */
	struct completion s_kobj_unregister;

	/* fscache support */
	struct fscache_volume *volume;
	struct erofs_fscache *s_fscache;
	struct erofs_domain *domain;
	char *fsid;
	char *domain_id;
};

bdev.c 中负责硬盘块操作

申请super_block

erofs_fc_fill_super 其实是从硬盘上读取数据，初始化user的sb信息

#inode 操作

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const struct inode_operations erofs_generic_iops = {
	.getattr = erofs_getattr,
	.listxattr = erofs_listxattr,
	.get_inode_acl = erofs_get_acl,
	.fiemap = erofs_fiemap,
};

const struct inode_operations erofs_symlink_iops = {
	.get_link = page_get_link,
	.getattr = erofs_getattr,
	.listxattr = erofs_listxattr,
	.get_inode_acl = erofs_get_acl,
};

const struct inode_operations erofs_fast_symlink_iops = {
	.get_link = simple_get_link,
	.getattr = erofs_getattr,
	.listxattr = erofs_listxattr,
	.get_inode_acl = erofs_get_acl,
};

#erofs-utils

  • mkfs.erofs 创建一个 erofs 文件系统；
  • fsck.erofs 检查 erofs 镜像的完整性；
  • dump.erofs 导出 erofs 镜像中的相关细节；
  • erofsfuse 用户态

#mkfs

#fsck

#dump

#erofsfuse