OverlayFS联合文件系统源码解析系列—目录接口详解

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引言

在前篇中介绍了Linux是如何挂载Overlay文件系统的，重点关注了Overlay的挂载流程的实现和关键数据结构的关系，而在本文中主要介绍Overlay FS的目录相关的关键数据结构和目录的相关接口，其中比较复杂的接口将会在下一篇中展开介绍。

一、Overlay 关键数据结构

1、目录文件

struct ovl_dir_file {
    bool is_real;
    bool is_upper;
    struct ovl_dir_cache *cache;
    struct list_head *cursor;
    struct file *realfile;
    struct file *upperfile;
};

2、目录接口

// fs/overlayfs/readdir.c
const struct file_operations ovl_dir_operations = {
    .read       = generic_read_dir,
    .open       = ovl_dir_open,
    .iterate    = ovl_iterate,
    .llseek     = ovl_dir_llseek,
    .fsync      = ovl_dir_fsync,
    .release    = ovl_dir_release,
};

3、目录inode操作

// fs/overlayfs/dir.c
const struct inode_operations ovl_dir_inode_operations = {
    .lookup     = ovl_lookup,   // fs/overlayfs/namei.c
    .mkdir      = ovl_mkdir,    // fs/overlayfs/dir.c
    .symlink    = ovl_symlink,
    .unlink     = ovl_unlink,
    .rmdir      = ovl_rmdir,
    .rename     = ovl_rename,
    .link       = ovl_link,
    .setattr    = ovl_setattr,
    .create     = ovl_create,
    .mknod      = ovl_mknod,
    .permission = ovl_permission,
    .getattr    = ovl_getattr,
    .listxattr  = ovl_listxattr,
    .get_acl    = ovl_get_acl,
    .update_time    = ovl_update_time,
};

二、目录操作

1、创建目录

（1）创建目录系统调用

// ./fs/namei.c
SYSCALL_DEFINE2(mkdir, const char __user *, pathname, umode_t, mode)
|-> do_mkdirat(AT_FDCWD, pathname, mode);
    |-> struct dentry *dentry;
    |-> struct path path;
    | // (1)创建目录项：创建ovl_dentry，同时将其加入到其父目录的子目录项链表中
    |-> dentry = user_path_create(dfd, pathname, &path, lookup_flags); // path保存父目录路径；dentry保存新建目录项内存对象
    |            |-> filename_create(dfd, getname(pathname), path, lookup_flags);
    |                |-> name = filename_parentat(dfd, name, lookup_flags, path, &last, &type);
    |                |-> dentry = __lookup_hash(&last, path->dentry, lookup_flags)
    |                             |-> dentry = d_alloc(base, name);
    |
    | // (2)创建ovl inode：调用对应文件系统目录ops中的mkdir创建inode，并关联inode到dentry
    |-> vfs_mkdir(path.dentry->d_inode, dentry, mode);
        |-> dir->i_op->mkdir(dir, dentry, mode) // 参数：父目录inode、新目录的目录项、模式

（2）Overlay FS中ovl_mkdir创建目录的过程

在Overlay fs联合文件系统下，创建目录有2种情况：
（1）upper层和lower层均无同名目录，则直接在upper层创建。
（2）lower层有同名目录，但是之后被删除了，则需要在已经存在whiteout文件的情况下创建目录。

ovl_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode) // fs/overlayfs/dir.c
|-> ovl_create_object(dentry, (mode & 07777) | S_IFDIR, 0, NULL)
    |   // （1）创建ovl_inode节点
    |-> inode = ovl_new_inode(dentry->d_sb, mode, rdev);
    |-> inode->i_state |= I_CREATING;
    |
    |   // （2）联合文件系统调用（实际文件系统）创建节点，并连接（link）虚拟文件系统inode和实际文件系统inode
    |   // 实际文件系统创建新目录，并将实际文件系统的inode赋值给虚拟文件系统的目录项（dentry），同时虚拟文件系统的目录项作为虚拟文件系统目录项的一个别名
    |-> ovl_create_or_link(dentry, inode, &attr, false) // 参数：虚拟文件系统目录项、虚拟文件系统inode节点、属性、非原始节点
        | // 父目录进行copyup：将parent的lower目录内容copyup到upper层。若存在upper层，则直接返回。
        |-> ovl_copy_up(parent);// copyup将会在后续的文章中详细介绍
        |-> if (!ovl_dentry_is_whiteout(dentry))
        |       // 不存在whiteout文件，则直接在upper层创建新目录
        |->     ovl_create_upper(dentry, inode, attr);
        |-> else
        |       // 存在whiteout文件，则在upper层创建新目录并..???..
        |->     ovl_create_over_whiteout(dentry, inode, attr);

（3）直接在upper层创建新目录

参数inode为ovl_inode，而dentry则是与之对应的overlayfs文件系统目录节点的目录项。
ovl_create_upper(struct dentry *dentry, struct inode *inode, struct ovl_cattr *attr)
| // 将overlayfs目录转换成源文件路径
|-> upperdir = ovl_dentry_upper(dentry->d_parent); // 原始实际文件系统中的父目录
|-> struct inode *udir = upperdir->d_inode;
|-> struct dentry *newdentry;
| // （1）创建父目录的upper层创建新目录
|-> newdentry = ovl_create_real(udir, lookup_one_len(dentry->d_name.name, upperdir, dentry->d_name.len), attr);
|               |-> ovl_mkdir_real(dir, &newdentry, attr->mode);
|                   |-> ovl_do_mkdir(dir, dentry, mode)
|                       |-> vfs_mkdir()
|                           |-> dir->i_op->mkdir(dir, dentry, mode); 
|
| // 实例化overlayfs目录项和ovl_inode:
| // (1)ovl_inode的i_private和__upperdentry分别指向upper的inode和目录项;(2)ovl目录项的d_inode指向ovl_inode
|-> ovl_instantiate(dentry, inode, newdentry, !!attr->hardlink); // ovl: dentry; ovl: inode; realfs: newdentry, ..
    |-> struct ovl_inode_params oip = {
    |      .upperdentry = newdentry, // 实际文件系统目录项
    |      .newinode = inode, // ovl_inode
    |   };
    |
    | // 返回ovl_inode的指针：ovl_inode的i_private和__upperdentry分别指向upper的inode和目录项
    |-> inode = ovl_get_inode(dentry->d_sb, &oip);
    |           | // 返回ovl_inode的指针,并将realinode作为ovl_inode私有数据
    |           |-> inode = ovl_iget5(sb, oip->newinode, key);
    |           | // 设置ovlinode的__upperdentry指向upper的目录项upperdentry
    |           |-> ovl_inode_init(inode, oip, ino, fsid);
    |               |-> OVL_I(inode)->__upperdentry = oip->upperdentry;
    |
    | // 设置ovl目录项的d_inode指向ovl_inode
    |-> d_instantiate(dentry, inode); // overlayfs文件系统目录项和inode
        |-> __d_instantiate(entry, inode);
            |-> __d_set_inode_and_type(dentry, inode, add_flags);
            |-> dentry->d_inode = inode;

ovl_create_upper()执行完后，新创建的overlayfs目录结构，如下图所示：

overlayfs:                                       struct ovl_inode                       
           struct dentry                         +---------------------------------+    
           +--------------------------------+    |redirect: char*;                 |    
           |d_parent: struct dentry *;      |    |mnt_mountpoint: struct dentry*;  |    
           |d_name: struct qstr;            |    |vfs_inode: struct inode;         |    
           |d_inode: struct inode*;  -------+--->|+-struct inode -----------------+|    
           |d_op: struct dentry_operations*;|    ||i_op: struct inode_operations*;||    
           |d_sb: struct super_block;       |    ||i_sb: struct super_block *;    ||    
           |d_fsdata: void *;               |    || ...                           ||    
           | ...                            |    ||i_private: void *  ------------++---+
           +--------------------------------+    |+-------------------------------+|   |
                                                 |__upperdentry: struct dentry*; --+-+ |
                                                 |lower: struct inode*;            | | |
                                                 |...                              | | |
                                                 +---------------------------------+ | |
                                                                                     | |
upper:     +-------------------------------------------------------------------------+ |
           |                                     +-------------------------------------+
           |                                     |                                      
           V                                     V                                      
           struct dentry                         struct inode                
           +--------------------------------+ +->+-------------------------------+      
           |d_parent: struct dentry *;      | |  |i_op: struct inode_operations*;|      
           |d_name: struct qstr;            | |  |i_sb: struct super_block *;    |      
           |d_inode: struct inode*; --------+-+  |i_ino: unsigned long;          |      
           |d_op: struct dentry_operations*;|    | ...                           |      
           |d_sb: struct super_block;       |    +-------------------------------+      
           | ...                            |                                           
           +--------------------------------+

（4）创建新目录，但lower层存在被删除的同名目录

参数ovl_create_upper()一样：inode为ovl_inode，而dentry则是与之对应的overlayfs文件系统目录节点的目录项。
ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode, struct ovl_cattr *attr)
|-> struct dentry *workdir = ovl_workdir(dentry);
|-> struct inode *wdir = workdir->d_inode;
|-> struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent);   // upper父目录项
|-> struct inode *udir = upperdir->d_inode;                         // upper父目录inode
|-> struct dentry *upper;                                           // ovl目录项
|-> struct dentry *newdentry;                                       // upper目录项
|
| // (1)在upper层父目录中查找目录项（whiteout）
|-> upper = lookup_one_len(dentry->d_name.name, upperdir, dentry->d_name.len);
| // (2)在workdir目录下创建临时目录
|-> newdentry = ovl_create_temp(workdir, cattr);
|               |-> ovl_create_real(d_inode(workdir), ovl_lookup_temp(workdir), attr);
| // (3)设置worddir下新建临时目录属性为opaque
|-> ovl_set_opaque(dentry, newdentry);
| // (4)workdir新建的临时目录与upper的whiteout目录做交互rename
|-> ovl_do_rename(wdir, newdentry, udir, upper, RENAME_EXCHANGE);
| // (5)清除rename后workdir下的废弃文件（whiteout）
|-> ovl_cleanup(wdir, upper);
|
| // 实例化overlayfs目录项和ovl_inode：
| // (1)ovl_inode的i_private和__upperdentry分别指向upper的inode和目录项;(2)ovl目录项的d_inode指向ovl_inode.
|-> ovl_instantiate(dentry, inode, newdentry, !!attr->hardlink);

由上面流程可见，与ovl_create_upper()相比，ovl_create_over_whiteout()先在work目录下创建一个临时目录，并设置目录属性为opaque，以屏蔽lower层同名目录。然后将workdir目录下临时目录与upper层的whiteout文件做重命名互换，并清除互换后的垃圾文件。

2、删除目录ovl_rmdir()

将要被删除的联合文件系统目录主要有3种途径：
（1）目录只来自upper层，则直接删除即可；
（2）目录只来自lower层，则需要建立对lower层同名目录的whiteout文件；
（3）目录是由uppwer层和lower层目录合成，则既要删除upper层目录，又要建立lower层同名目录的whiteout文件；

但是上面的情况，经过简化合并为两种情况：

不存在于lower层，则直接删除即可
存在于lower层，需要生成whiteout文件屏蔽底层目录。

下面代码为Overlay fs联合文件系统删除目录的过程，留意下列过程中根据lower_positive是否为true，即是否存在lower层，分别调用了函数ovl_remove_upper()或者ovl_remove_and_whiteout()。

ovl_rmdir(struct inode *dir, struct dentry *dentry)
|-> ovl_do_remove(dentry, true);
    |-> ovl_check_empty_dir(dentry, &list);
    |   | // 获取目录下的所有目录和文件列挂到list链表
    |   |-> ovl_dir_read_merged(dentry, list, &root);
    |   |   |-> for (idx = 0; idx != -1; idx = next) {
    |   |   |->     next = ovl_path_next(idx, dentry, &realpath);
    |   |   |->     ovl_dir_read(&realpath, &rdd);
    |   |   |        |-> realfile = ovl_path_open(realpath, O_RDONLY | O_LARGEFILE);
    |   |   |        |-> iterate_dir(realfile, &rdd->ctx);
    |   |   |-> }
    |   |-> 
    |
    | // copyup父目录：若存在upper层，则ovl_copy_up()直接返回。
    |-> ovl_copy_up(dentry->d_parent);
    |-> if (!lower_positive)
    |->     err = ovl_remove_upper(dentry, is_dir, &list);
    |-> else
    |->     err = ovl_remove_and_whiteout(dentry, &list);
    |
    |-> clear_nlink(dentry->d_inode);
    |
    |-> upperdentry = ovl_dentry_upper(dentry);
    |-> if (upperdentry)
    |->     ovl_copyattr(d_inode(upperdentry), d_inode(dentry));

（1）删除不存在lower层的目录

ovl_remove_upper(dentry, is_dir, &list)
|-> struct dentry *upperdir = ovl_dentry_upper(dentry->d_parent)
|-> struct inode *dir = upperdir->d_inode
|-> struct dentry *opaquedir = NULL
|-> opaquedir = ovl_clear_empty(dentry, list) // 返回opaque目录
|-> upper = ovl_lookup_upper(ofs, dentry->d_name.name, upperdir,dentry->d_name.len)// 找到需要删除的dengtry
|-> err = ovl_do_rmdir(ofs, dir, upper)
| |-> vfs_rmdir(ovl_upper_mnt_userns(ofs), dir, dentry) // 直接调用vfs_mkdir进行删除
| |...

（2）删除存在lower层的目录

ovl_remove_and_whiteout(dentry, &list);
|-> struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
|-> workdir = ovl_workdir(dentry);
| // 返回opaque目录：已经完成了目标目录的删除
|-> opaquedir = ovl_clear_empty(dentry, list);
|               |-> opaquedir = ovl_create_temp(workdir, OVL_CATTR(stat.mode));
|               |-> ovl_copy_xattr(dentry->d_sb, upper, opaquedir);
|               |-> ovl_set_opaque(dentry, opaquedir);
|               | // rename后opaquedir与upper的inode内容互换
|               |-> ovl_do_rename(wdir, opaquedir, udir, upper, RENAME_EXCHANGE);
|               | // 递归删除目标目录下的upper层内容
|               |-> ovl_cleanup_whiteouts(upper, list);
|               | // 删除目标目录
|               |-> ovl_cleanup(wdir, upper);
|                   |-> ovl_do_rmdir(wdir, wdentry);
|                       |-> vfs_rmdir(dir, dentry);
| // 此时，upper即opaquedir
|-> upper = lookup_one_len(dentry->d_name.name, upperdir, dentry->d_name.len);
|-> ovl_cleanup_and_whiteout(ofs, d_inode(upperdir), upper);
    |-> whiteout = ovl_whiteout(ofs);
    |              |-> whiteout = ovl_lookup_temp(workdir);
    |              |              |-> snprintf(name, sizeof(name), "#%x", atomic_inc_return(&temp_id));
    |              |              |-> temp = lookup_one_len(name, workdir, strlen(name));
    |              |-> ovl_do_whiteout(wdir, whiteout);
    |-> ovl_do_rename(wdir, whiteout, dir, dentry, flags);
    |-> ...

如果要删除的文件是upper层覆盖lower层的文件，要删除的目录是上下层合并的目录，其实是前两个场景的合并，Overlay fs即需要删除upper层对应文件系统中的文件或目录，也需要在对应位置创建同名whiteout文件，让upper层的文件被删除后不至于lower层的文件被暴露出来。

3、ovl_dir_open打开目录

ovl_dir_open(struct inode *inode, struct file *file) // fs/overlayfs/readdir.c
|-> struct ovl_dir_file *od;
|-> od = kzalloc(sizeof(struct ovl_dir_file), GFP_KERNEL);
| // 获取目录路径：若目录是merged或者upper目录，则获得upper路径；否则，获得lower路径。
|-> type = ovl_path_real(file->f_path.dentry, &realpath);
|          |-> type = ovl_path_type(dentry);
|          |          |-> type = __OVL_PATH_UPPER;
|          |          |-> type  |= __OVL_PATH_MERGE;
|          |
|          |-> ovl_path_lower(dentry, path);
|          | // < or >
|          |-> ovl_path_upper(dentry, path);
|              |-> path->mnt = ovl_upper_mnt(ofs);
|              |-> path->dentry = ovl_dentry_upper(dentry);
|
| // 打开upper或者lower目录
|-> realfile = ovl_dir_open_realfile(file, &realpath);
|              |-> struct file *res;
|              |-> res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
|                        |-> dentry_open(path, flags, current_cred());
|                            |-> struct file *f;
|                            |-> f = alloc_empty_file(flags, cred);
|                            |-> vfs_open(path, f);
|                                |-> open = f->f_op->open(inode, f);
|
| // realfile作为ovldir_file的实际文件；ovl_dir_file又作为file的私有数据
|-> od->realfile = realfile;
|-> od->is_real = ovl_dir_is_real(file->f_path.dentry);
|-> od->is_upper = OVL_TYPE_UPPER(type);
|-> file->private_data = od;

目录打开后形成如下结构：

struct file                      
           +------------------------------+ 
           |f_path: struct path;          | 
           |f_inode: struct inode;        | 
           |f_op: struct file_operations*;| 
           |private_data: void *; --------+-+
           | ...                          | |
           +------------------------------+ |
                                    +-------+
Overlay fs:                          |  struct ovl_dir_file                 
                                    +->+-----------------------------+
                                       |is_real: bool                |
                                       |is_upper: unsigned;          |
                                       |cache: struct ovl_dir_cache*;|
                                       |cursor: struct list_head*;   |
                                       |realfile: struct file*; -----+-+
                                       |upperfile: struct file*;     | |
                                       +-----------------------------+ |
                                                              +--------+
                                                              |                                 
                                                              |  struct file                     
upper:                                                        +->+------------------------------+
                                                                 |f_path: struct path;          |
                                                                 |f_inode: struct inode;        |
                                                                 |f_op: struct file_operations*;|
                                                                 |private_data: void *;         |
                                                                 | ...                          |
                                                                 +------------------------------+

每当在这样一个合并的目录中请求查找时，就会在每个实际的目录中进行查找，而合并的结果则缓存在属于覆盖文件系统的目录中。结果被缓存在属于叠加文件系统的目录中。如果两个实际的查询都能找到目录，那么这两个查询都会被存储起来，并且创建一个合并的目录。创建一个合并的目录，否则只存储一个：如果上层目录存在，则存储下层目录。只有来自目录的名称列表被合并。其他内容，如元数据和扩展属性，只报告给上层目录，下层目录的这些属性被隐藏。

/* 
  DESCRIPTION: 在特定文件系统中，在目录dir下查找dentry中包含
  PARAMS:
    @dir: 父目录，在该目录下查找目标目录或者文件，但是函数内未使用，父目录由dentry的d_parent获得。
    @dentry: 目标目录或者文件的目录项，包含name，但是缺少inode
    @flags*/
ovl_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags) // fs/overlayfs/dir.c
| // (1)在upper层父目录中查找目标文件或者目录：
| // (1.1)从ovl目录项的ovl_inode反向索引到实际文件系统的dentry
|-> upperdir = ovl_dentry_upper(dentry->d_parent);
| // (1.2)在upper层目录下搜索目标文件或者目录，若存在则使用局部目录项指针upperdentry指向目标目录项
|-> ovl_lookup_layer(upperdir, &d, &upperdentry, true); // 参数：实际文件系统中的父目录项、...、目标实际文件系统中目录项返回值、...
|   |-> ovl_lookup_single(base, d, d->name.name, d->name.len,0, "", ret, drop_negative) // ret，即&upperdentry
|       |-> this = ovl_lookup_positive_unlocked(name, base, namelen, drop_negative);
|       |          |-> lookup_one_len_unlocked(name, base, len);
|       |              |-> lookup_slow(&this, base, 0); 
|       |                   |-> __lookup_slow(name, dir, flags);
|       |                       |-> dentry = d_alloc_parallel(dir, name, &wq);
|       |                       |-> old = inode->i_op->lookup(inode, dentry, flags);
|       |                       |-> dentry = old;
|       |-> *ret = this;
|
| // (2)在所有lower层父目录中查找目标文件或者目录
|-> for (i = 0; !d.stop && i < poe->numlower; i++) {
|->     struct ovl_path lower = poe->lowerstack[i];
|->     d.last = i == poe->numlower - 1;
|->     ovl_lookup_layer(lower.dentry, &d, &this, false);
|->     ...
|->     stack[ctr].dentry = this;
|->     stack[ctr].layer = lower.layer;
|->     ctr++;
|-> ｝
|
|   // (3)分配ovl_entry，其中包括根据堆叠层次确定的ovl_path层次数
|-> oe = ovl_alloc_entry(ctr);
|-> memcpy(oe->lowerstack, stack, sizeof(struct ovl_path) * ctr);
|-> dentry->d_fsdata = oe;
|
|   // 关联实际文件系统denty和inode到虚拟文件系统inode
|-> inode = ovl_get_inode(dentry->d_sb, &oip);

根据上面的流程，在overlayfs文件系统的一个目录下查询一个目录或者文件的流程为，通过ovl系统中父目录dentry找到实际文件系统的目录项和inode，找到目标目录或者文件对应的dentry和inode。

struct ovl_entry                 
                                     +==>+-------------------------------+
                                     H   |lowerstack: struct ovl_path[]; |
                                     H   |numlower: unsigned;            |
                                     H   | ...                           |
                                     H   +-------------------------------+
                                     H
                                     H   struct ovl_inode                   
   struct dentry(d_parent)           H   +---------------------------------+       
   +--------------------------------+H   |redirect: char*;                 |           
   |d_parent: struct dentry *;      |H   |mnt_mountpoint: struct dentry*;  |                  
   |d_name: struct qstr;            |H   |vfs_inode: struct inode;         |               
   |d_inode: struct inode*;==========H==>|+-struct inode -----------------+|                    
   |d_op: struct dentry_operations*;|H   ||i_op: struct inode_operations*;||                    
   |d_sb: struct super_block;       |H   ||i_sb: struct super_block *;    ||                
   |d_fsdata: void *;  ==========(2)=H   ||i_ino: unsigned long;          ||
   | ...                            |    || ...                           ||
   +--------------------------------+    |+-------------------------------+|      struct dentry(base dir in realfs)      
                                         |__upperdentry: struct dentry*; ==(1)===>+--------------------------------+      
                                         |lower: struct inode*;            |      |d_parent: struct dentry *;      |     
                                         |...                              |      |d_name: struct qstr;            |                                    
                                         +---------------------------------+      |d_inode: struct inode*; ============H                                  
                                                                                  |d_op: struct dentry_operations*;|   H                                            
                                                                                  |d_sb: struct super_block;       |   H
                                                                                  | ...                            |   H
                                                                                  +--------------------------------+   H
                                                                                       struct inode(lowerendtry->inode)V
                                                                                        +-------------------------------+
                                                                                        |i_op: struct inode_operations*;| ==> 实际文件系统的lookup()
                                                                                        |i_sb: struct super_block *;    |
                                                                                        |i_ino: unsigned long;          |
                                                                                        | ...                           |
                                                                                        +-------------------------------+

查找到的目录或者文件，在ovl文件系统中表示如下：

struct ovl_entry                 
                                     +->+------------------------------+
                                     |  | ...                          |
                                     |  |numlower: unsigned;           |
                                     |  |lowerstack: struct ovl_path[];|
                                     |  +==============================+
                                     |  |struct ovl_path {             |
                                     |  |  layer: struct ovl_layer*;   |
                                     |  |  dentry: struct dentry*;     |
                                     |  |};                            |
                                     |  +------------------------------+
                                     |  |struct ovl_path {             |
                                     |  |  layer: struct ovl_layer*;   |
                                     |  |  dentry: struct dentry*;     |
                                     |  |};                            |
                                     |  +------------------------------+
                                     |  |                              |
                                     |  |                              |
                                     |
                                     |  struct ovl_inode                   
   struct dentry                     |  +---------------------------------+       
   +--------------------------------+|  |redirect: char*;                 |           
   |d_parent: struct dentry *;      ||  |mnt_mountpoint: struct dentry*;  |                  
   |d_name: struct qstr;            ||  |vfs_inode: struct inode;         |               
   |d_inode: struct inode*;----------+->|+-struct inode -----------------+|                    
   |d_op: struct dentry_operations*;||  ||i_op: struct inode_operations*;||                    
   |d_sb: struct super_block;       ||  ||i_sb: struct super_block *;    ||                
   |d_fsdata: void *;  -(3)----------+  ||i_ino: unsigned long;          ||
   | ...                            |   || ...                           ||
   +--------------------------------+   |+-------------------------------+|
                                        |__upperdentry: struct dentry*;   |
                                        |lower: struct inode*;            |
                                        |...                              |                              
                                        +---------------------------------+

（2）打开目录

被打开的目录可能只来自于upper层或lower层，由或者是upper和lower目录merged而成，打开过程如下：

ovl_dir_open(struct inode *inode, struct file *file) // fs/overlayfs/readdir.c
|-> struct ovl_dir_file *od;
|-> od = kzalloc(sizeof(struct ovl_dir_file), GFP_KERNEL);
| // （1）获取目录路径：若目录是merged或者upper目录，则获得upper路径；否则，获得lower路径。
|-> type = ovl_path_real(file->f_path.dentry, &realpath);
|          |-> type = ovl_path_type(dentry);
|          |          |-> type = __OVL_PATH_UPPER;
|          |          |-> type  |= __OVL_PATH_MERGE;
|          |
|          |-> ovl_path_lower(dentry, path);
|          | // < or >
|          |-> ovl_path_upper(dentry, path);
|              |-> path->mnt = ovl_upper_mnt(ofs);
|              |-> path->dentry = ovl_dentry_upper(dentry);
|
| // （2）打开实际文件系统目录文件：upper或者lower目录
|-> realfile = ovl_dir_open_realfile(file, &realpath);
|              |-> struct file *res;
|              |-> res = ovl_path_open(realpath, O_RDONLY | (file->f_flags & O_LARGEFILE));
|                        |-> dentry_open(path, flags, current_cred());
|                            |-> struct file *f;
|                            |-> f = alloc_empty_file(flags, cred);
|                            |-> vfs_open(path, f);
|                                |-> open = f->f_op->open(inode, f);
|
| // （3）将overlayfs目录文件和实际文件系统目录文件关联：realfile作为ovldir_file的实际文件；ovl_dir_file又作为file的私有数据
|-> od->realfile = realfile;
|-> od->is_real = ovl_dir_is_real(file->f_path.dentry);
|-> od->is_upper = OVL_TYPE_UPPER(type);
|-> file->private_data = od;

由上过程可知，Overlay fs目录文件被打开后，同时存在Overlay fs目录文件和实际文件系统目录文件，中间通过ovl_dir_file链接（ovl_dir_file作为Overlay fs目录文件的私有数据），他们直接的关系示意如下：

struct ovl_dir_file                 
                            +->+-----------------------------+
                            |  |is_real: bool                |
                            |  |is_upper: unsigned;          |
                            |  |cache: struct ovl_dir_cache*;|
                            |  |cursor: struct list_head*;   |
                            |  |realfile: struct file*; -----+-+
                            |  |upperfile: struct file*;     | |
                            |  +-----------------------------+ |
                            +-------+                  +-------+
   struct file                      |                  |  struct file                     
   +------------------------------+ |                  +->+------------------------------+  
   |f_path: struct path;          | |                     |f_path: struct path;          |         
   |f_inode: struct inode;        | |                     |f_inode: struct inode;        |     
   |f_op: struct file_operations*;| |                     |f_op: struct file_operations*;|
   |private_data: void *; --------+-+                     |private_data: void *;         |
   | ...                          |                       | ...                          |
   +------------------------------+                       +------------------------------+
   overlayfs目录文件                                      实际文件系统目录文件

（3）遍历目录

遍历目录，即是所有各叠层相同目录下的所有文件列表。

（4）系统调用-遍历目录

YSCALL_DEFINE3(getdents, unsigned int, fd,,,,,) // fs/readdir.c
|-> struct fd f;
|-> struct getdents_callback buf = {
|->     .ctx.actor = filldir,
|->     .count = count,
|->     .current_dir = dirent
|-> };
|
|-> iterate_dir(f.file, &buf.ctx);
    |-> file->f_op->iterate(file, ctx);

（5）OVL函数iterate实现

struct ovl_dir_file                 
                                    +=>+-----------------------------+
                                    H  |is_real: bool                |
                                    H  |is_upper: unsigned;          |
                                    H  |cache: struct ovl_dir_cache*;|
                                    H  |cursor: struct list_head*;   |
                                    H  |realfile: struct file*; -----+-+
                                    H  +-----------------------------+ |
                                    H                                  | 
   struct file                      H    struct file                   V  
   +------------------------------+ H    +------------------------------+  
   |f_path: struct path;  ==========+=+  |f_path: struct path;          |         
   |f_inode: struct inode;        | H H  |f_inode: struct inode;        |     
   |f_op: struct file_operations*;| H H  |f_op: struct file_operations*;|
   |private_data: void *; ========+=+ H  |private_data: void *;         |
   | ...                          |   H  | ...                          |
   +------------------------------+   H  +------------------------------+
     +================================+
     V
    struct path                     
    +-----------------------+
    |mnt: struct vfsmount*; |
    |dentry: struct dentry*;|==+
    +-----------------------+  H
     +=========================+
     V
     struct dentry                        struct ovl_inode                   
     +--------------------------------+   +---------------------------------+
     |d_parent: struct dentry *;      |   |cache: struct ovl_dir_cache*;    |
     |d_name: struct qstr;            |   |flags: unsigned long;            |
     |d_inode: struct inode*;=========+==>|vfs_inode: struct inode;         |
     |d_op: struct dentry_operations*;|   |+-struct inode -----------------+|
     |d_sb: struct super_block;       |   ||i_op: struct inode_operations*;||
     |d_fsdata: void *;  =============++  ||i_sb: struct super_block *;    ||
     | ...                            |H  ||i_ino: unsigned long;          ||
     +--------------------------------+H  || ...                           ||
                                       H  |+-------------------------------+|
                                       H  |__upperdentry: struct dentry*;   |
                                       H  |...                              |
                                       H  +---------------------------------+
                                       H                  
                                       H  struct ovl_entry                
                                       +=>+------------------------------+
                                          | ...                          |
                                          |numlower: unsigned;           |
                                          |lowerstack: struct ovl_path[];|
                                          +==============================+
                                          |struct ovl_path {             |
                                          |  layer: struct ovl_layer*;   |
                                          |  dentry: struct dentry*;     |
                                          |};                            |
                                          +------------------------------+
                                          |                              |
                                          |                              |

ovl_iterate(struct file *file, struct dir_context *ctx)
|-> struct ovl_dir_file *od = file->private_data;
|-> struct ovl_dir_cache *cache;
|
|-> cache = ovl_cache_get(dentry);
|           |-> ovl_set_dir_cache(d_inode(dentry), NULL);
|           |-> cache = kzalloc(sizeof(struct ovl_dir_cache), GFP_KERNEL);
|           |-> cache->refcount = 1;
|           |-> INIT_LIST_HEAD(&cache->entries);
|           |-> cache->root = RB_ROOT;
|           |  // 遍历同名目录所在所有层次的文件列表，挂载文件列表到cache的红黑树节点
|           |-> ovl_dir_read_merged(dentry, &cache->entries, &cache->root);
|           |   |-> for (idx = 0; idx != -1; idx = next) {
|           |   |       next = ovl_path_next(idx, dentry, &realpath);
|           |   |              |-> struct ovl_entry *oe = dentry->d_fsdata;
|           |   |              |-> if (idx == 0) ovl_path_upper(dentry, path);
|           |   |              |        
|           |   |              |-> path->dentry = oe->lowerstack[idx - 1].dentry;
|           |   |       if (next != -1) {
|           |   |           // 遍历某一层中目录下文件列表，结果存放在ctx中
|           |   |           err = ovl_dir_read(&realpath, &rdd);
|           |   |                 |-> realfile = ovl_path_open(realpath, O_RDONLY | O_LARGEFILE);
|           |   |                 |-> iterate_dir(realfile, &rdd->ctx);
|           |   |       } else {
|           |   |           err = ovl_dir_read(&realpath, &rdd);
|           |   |       }
|           |   |-> }
|           |-> cache->root = RB_ROOT;
|           |-> ovl_set_dir_cache(d_inode(dentry), cache);
|
|-> od->cache = cache;
|-> ovl_seek_cursor(od, ctx->pos);
|   |-> od->cursor = p;

由上面的流程可以看出，若是目录是合成的，在遍历目录下文件列表时，是将各叠层目录下的所有文件列表保存在文件所有的红黑数上，结构如下：

struct ovl_dir_file                                
                                    +=>+-----------------------------+               
                                    H  |is_real: bool                |               
                                    H  |is_upper: unsigned;          |               
                                    H  |cache: struct ovl_dir_cache*;+--------+      
                                    H  |cursor: struct list_head*;   |        |      
                                    H  |realfile: struct file*; -----+-+      |        
                                    H  +-----------------------------+ |      |        
                                    H                                  |      |         
   struct file                      H    struct file                   V      |          
   +------------------------------+ H    +------------------------------+     |           
   |f_path: struct path;  ==========+=+  |f_path: struct path;          |     |                  
   |f_inode: struct inode;        | H H  |f_inode: struct inode;        |     |              
   |f_op: struct file_operations*;| H H  |f_op: struct file_operations*;|     |         
   |private_data: void *; ========+=+ H  |private_data: void *;         |     |                         保存所有不同层次相同目录下的文件列表（红黑树）
   | ...                          |   H  | ...                          |     |                        /
   +------------------------------+   H  +------------------------------+     |                       V
     +================================+                                       |  struct ovl_dir_cache                                    
     V                                                                        +->+------------------------+               
    struct path                                                               |  |refcount: long;         |               
    +-----------------------+                                                 |  |version: u64;           |               
    |mnt: struct vfsmount*; |                                                 |  |entries: struct entries;|               
    |dentry: struct dentry*;|==+                                              |  |root: struct rb_root;   |               
    +-----------------------+  H                                              |  +------------------------+               
     +=========================+                                              |                
     V                                                                        |                
     struct dentry                        struct ovl_inode                    |                
     +--------------------------------+   +---------------------------------+ |             
     |d_parent: struct dentry *;      |   |cache: struct ovl_dir_cache*; ---+-+              
     |d_name: struct qstr;            |   |flags: unsigned long;            |
     |d_inode: struct inode*;=========+==>|vfs_inode: struct inode;         |
     |d_op: struct dentry_operations*;|   |+-struct inode -----------------+|
     |d_sb: struct super_block;       |   ||i_op: struct inode_operations*;||
     |d_fsdata: void *;               |   ||i_sb: struct super_block *;    ||
     | ...                            |   ||i_ino: unsigned long;          ||
     +--------------------------------+   || ...                           ||
                                          |+-------------------------------+|
                                          |__upperdentry: struct dentry*;   |
                                          |...                              |
                                          +---------------------------------+

总结

本文重点介绍Overlay FS的目录相关的关键数据结构和目录的相关接口，让读者能体会到所谓的Overlay文件系统的特点，很多实现细节是有别于传统的文件系统，均需考虑upper层和lower层不同情况，但是万变不离其宗，我们学习文件系统必须抓住文件系统的主要数据和接口的实现，这对C语言实现其它底层软件开发也是一种必备技能。