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在前篇中介绍了Linux是合文如何挂载Overlay文件系统的，重点关注了Overlay的系解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的统源目录相关的关键数据结构和目录的相关接口，其中比较复杂的码解接口将会在下一篇中展开介绍。
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;};// fs/overlayfs/readdir.cconst 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,};// fs/overlayfs/dir.cconst 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,};// ./fs/namei.cSYSCALL_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）<u>lower层有同名目录，但是之后被删除了</u>，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);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; | +-------------------------------+ | ... | +--------------------------------+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。
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：
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));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| |...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。
假设目录是merged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*; | |... | +---------------------------------+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遍历目录，即是所有各叠层相同目录下的所有文件列表。
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);假设目录是merged的目录：
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);| | | | <or>| | | |-> 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*; | |... | +---------------------------------+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。
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责任编辑：jianghua 来源： 51CTO 开源基础软件社区 OverlayC语言(责任编辑：焦点)
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