uname -r

# 查看是否已加載overlay 模塊
lsmod | grep overlay

# 若沒有加載則可手動加載
modprobe overlay

# 直接繼續(xù)overlay fs的掛載，自動加載overlay模塊
mount -t overlay overlay -o lowerdir=/lower,upperdir=/upper,workdir=/work  /merged

# 該命令指定一個lower層，一個upper層
sudo mount -t overlay overlay -o lowerdir=/lower,upperdir=/upper,workdir=/work /merged

# 若需要指定多個lower層，可以使用如下命令
sudo mount -t overlay overlay -o lowerdir=/lower1:/lower2:/lower3,upperdir=/upper,workdir=/work  /merged

SYSCALL_DEFINE5(mount,,,,,,,) // fs/namespace.c
|-> user_path_at(AT_FDCWD, dir_name, LOOKUP_FOLLOW, &path);
|-> do_mount(kernel_dev, dir_name, kernel_type, flags, options);
    |-> path_mount(dev_name, &path, type_page, flags, data_page);
        |-> do_new_mount()
            | // (1) 找到指定文件系統(tǒng)
            |-> type = get_fs_type(fstype);
            |           |-> find_filesystem(name, len)
            |           |-> for (p = &file_systems; *p; p = &(*p)->next)
            |           |-> strncmp((*p)->name, name, len) == 0
            |           |-> return p;
            |
            | // (2) 創(chuàng)建fc
            |-> fc = fs_context_for_mount(type, sb_flags);
            |        |-> alloc_fs_context(fs_type, NULL, sb_flags, 0, FS_CONTEXT_FOR_MOUNT);
            |                |-> fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
            |                |-> fc->fs_type    = get_filesystem(fs_type);
            |                |-> legacy_init_fs_context(fc);
            |                    |-> fc->fs_private = kzalloc(sizeof(struct legacy_fs_context), GFP_KERNEL);
            |                    |-> fc->ops = &legacy_fs_context_ops;
            |
            | // (3) 將fc的root指針指向overlayfs掛載點根root目錄
            |-> vfs_get_tree(fc)
            |   | // 通過fc的get_tree間接調(diào)用文件系統(tǒng)mount函數(shù)，以獲得掛載節(jié)點root目錄項（記錄在fc中）
            |   | // fc->ops->get_tree(fc); => legacy_get_tree
            |   |-> legacy_get_tree(fc) // 將掛載點的root目錄項賦值到fc的root
            |       |-> root = fc->fs_type->mount(fc->fs_type, fc->sb_flags, fc->source, ctx->legacy_data);
            |       |-> fc->root = root; // struct dentry *
            |
            | // (4) 創(chuàng)建mount實體，并將其掛載到supper_block的掛載鏈表中
            |-> do_new_mount_fc(fc, path, mnt_flags)；// path的形參即為mountpoint，是實際文件系統(tǒng)中mount掛載目標(biāo)路徑
                |-> mnt = vfs_create_mount(fc);
                |         |-> mnt = alloc_vfsmnt(fc->source ?: "none");
                |         |-> mnt->mnt.mnt_sb       = fc->root->d_sb;
                |         |-> mnt->mnt.mnt_root = dget(fc->root);
                |         |-> mnt->mnt_mountpoint   = mnt->mnt.mnt_root;
                |         |-> mnt->mnt_parent       = mnt;
                |         |-> list_add_tail(&mnt->mnt_instance, &mnt->mnt.mnt_sb->s_mounts);
                |
                |-> do_add_mount(real_mount(mnt), mp, mountpoint, mnt_flags);

ovl_mount() // fs/overlayfs/super.c
|-> mount_nodev(fs_type, flags, raw_data, ovl_fill_super); // return "dentry* s_root"
    | // (1) 為新掛載點創(chuàng)建superblock
    |-> s = sget() // struct super_block *
    |       |-> s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns); 
    |       |-> list_add_tail(&s->s_list, &super_blocks);
    |
    | // fill_super()
    |-> ovl_fill_super(s, data, flags & SB_SILENT ? 1 : 0)
        |-> struct ovl_fs *ofs; // private information held for overlayfs's superblock
        |-> struct ovl_layer *layers;
        |
        |-> ofs = kzalloc(sizeof(struct ovl_fs), GFP_KERNEL);
        |-> sb->s_d_op = &ovl_dentry_operations;
        |-> ovl_parse_opt((char *) data, &ofs->config);
        |-> numlower = ovl_split_lowerdirs(splitlower);
        |
        | // 創(chuàng)建聯(lián)合文件系統(tǒng)層級數(shù)組
        |-> layers = kcalloc(numlower + 1, sizeof(struct ovl_layer), GFP_KERNEL);
        |-> ofs->layers = layers;
        |
        |-> sb->s_op = &ovl_super_operations;
        |
        | // 設(shè)置upperlayer：設(shè)置layer的trap執(zhí)行upperpath目錄的inode；依據(jù)upperpath在實際文件系統(tǒng)中的掛載點填充layer的掛載點（ovl掛載的root為實際文件系統(tǒng)中一指定目錄）；
        |-> ovl_get_upper(sb, ofs, &layers[0], &upperpath);
        |   | // 獲得upperlayer目錄在實際文件系統(tǒng)上路徑：若路徑未打開過，則創(chuàng)建路徑上的所有節(jié)點的dentry與inode
        |   |-> ovl_mount_dir(ofs->config.upperdir, upperpath);
        |   | // 為掛載點創(chuàng)建一個inode，保存在trap中：新inode的private數(shù)據(jù)指向?qū)嶋H文件系統(tǒng)中掛載點目錄的inode
        |   |-> ovl_setup_trap(sb, upperpath->dentry, &upper_layer->trap,  "upperdir");
        |   | // 拷貝upper層實際文件系統(tǒng)路徑的掛載點為虛擬堆疊文件系統(tǒng)一個層次的新mount節(jié)點，且新的mount節(jié)點的root目錄為uppper目錄；
        |   |-> upper_mnt = clone_private_mount(upperpath);
        |   |-> upper_layer->mnt = upper_mnt;
        |
        | // 創(chuàng)建workdir: workbasedir為指定的實際文件系統(tǒng)內(nèi)的目錄，workbasedir_trap對workbasedir的inode的trap；workdir為workbasedir下創(chuàng)建的默認(rèn)名稱為“work”的目錄
        |-> ovl_get_workdir(sb, ofs, &upperpath);
        |   |-> ofs->workbasedir = dget(workpath.dentry);
        |   | // 申請一個ovl_inode，使其private指向?qū)嶋H文件系統(tǒng)inode，新對象地址保存在ofs->workbasedir_trap內(nèi)
        |   |-> ovl_setup_trap(sb, ofs->workbasedir, &ofs->workbasedir_trap, "workdir");
        |   |
        |   |-> ovl_make_workdir(sb, ofs, &workpath);
        |
        | // 逐個獲取lower層，檢查是否已經(jīng)有打上“trap”標(biāo)記的inode，同時為每個lower層的root inode打上“trap”標(biāo)記
        |-> oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers);
        |
        | // 對每個層對應(yīng)root inode的parent遞歸檢查是否已經(jīng)打上“trap”標(biāo)記，重疊層可能是互為祖先關(guān)系
        |-> ovl_check_overlapping_layers(sb, ofs);
        |
        |-> sb->s_fs_info = ofs;
        |
        |-> root_dentry = ovl_get_root(sb, upperpath.dentry, oe);
        |
        |-> sb->s_root = root_dentry;

ovl_get_upper(sb, ofs, &layers[0], &upperpath);
|-> struct vfsmount *upper_mnt;
| // （1）解析upper目錄：沿著用戶指定的upper目錄路徑解析，直至找到目錄的目錄項
|-> ovl_mount_dir(ofs->config.upperdir, upperpath);
|   |-> tmp = kstrdup(name, GFP_KERNEL);
|   |-> ovl_mount_dir_noesc(tmp, path);
|       |-> kern_path(name, LOOKUP_FOLLOW, path);
|           |-> filename_lookup(AT_FDCWD, getname_kernel(name), flags, path, NULL);
|
| // （2）設(shè)置upper_layer的trap：即從Overlayfs超級塊新申請一個inode，并將upper目錄的inode設(shè)置為其私有數(shù)據(jù)
|-> ovl_setup_trap(sb, upperpath->dentry, &upper_layer->trap,  "upperdir");
|   |-> trap = ovl_get_trap_inode(sb, dir);
|   |          |-> struct inode *key = d_inode(dir);
|   |          |-> struct inode *trap;
|   |          |-> trap = iget5_locked(sb, (unsigned long) key, ovl_inode_test, ovl_inode_set, key);
|   |          |-> return trap;
|   |-> *ptrap = trap;
|
| // （3）創(chuàng)建一個upper層目錄掛載節(jié)點clone體，并賦值給upper_layer的mnt成員
|-> upper_mnt = clone_private_mount(upperpath);
|               |-> struct mount *old_mnt = real_mount(path->mnt);
|               |-> struct mount *new_mnt;
|               | // 創(chuàng)建一個upper層的mount節(jié)點的clone體，并且將其掛載到upper層超級塊的掛載點鏈表上
|               |-> new_mnt = clone_mnt(old_mnt, path->dentry, CL_PRIVATE);
|               |             |-> struct super_block *sb = old->mnt.mnt_sb;
|               |             |-> struct mount *mnt;
|               |             |
|               |             |-> mnt = alloc_vfsmnt(old->mnt_devname);
|               |             |-> mnt->mnt.mnt_sb = sb;
|               |             |-> mnt->mnt.mnt_root = dget(root);
|               |             |-> list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
|               |
|               |-> return &new_mnt->mnt;
|-> upper_layer->mnt = upper_mnt;

static struct ovl_entry *
ovl_get_lowerstack(struct super_block *sb, const char *lower, unsigned int numlower, struct ovl_fs *ofs, struct ovl_layer *layers)
|-> struct path *stack = NULL;
|-> struct ovl_entry *oe;
|
| // （1）分配并初始化深度與lower層目錄樹對應(yīng)的path數(shù)組
|-> stack = kcalloc(numlower, sizeof(struct path), GFP_KERNEL);
|-> for (i = 0; i < numlower; i++) {
|       // 依次解析所有l(wèi)ower層目錄
|->     err = ovl_lower_dir(lower, &stack[i], ofs, &sb->s_stack_depth);
|-> }
|
| // 根據(jù)lower層目錄數(shù)組設(shè)置Overlayfs文件系統(tǒng)及其layers
|-> ovl_get_layers(sb, ofs, stack, numlower, layers);
|   |-> ofs->fs = kcalloc(numlower + 1, sizeof(struct ovl_sb), GFP_KERNEL);
|   |
|   |-> ofs->fs[0].sb = ovl_upper_mnt(ofs)->mnt_sb;
|   |-> ofs->fs[0].is_lower = false;
|   |
|   |-> for (i = 0; i < numlower; i++) {
|   |->     ovl_setup_trap(sb, stack[i].dentry, &trap, "lowerdir");
|   |->     mnt = clone_private_mount(&stack[i]);
|   |->     layers[ofs->numlayer].trap = trap;
|   |->     layers[ofs->numlayer].mnt = mnt;
|   |->     ofs->fs[fsid].is_lower = true;
|   |-> }
|
| // 設(shè)置overlayfs文件系統(tǒng)的根目錄ovl_entry
|-> oe = ovl_alloc_entry(numlower);
|-> for (i = 0; i < numlower; i++) {
|->     oe->lowerstack[i].dentry = dget(stack[i].dentry);
|->     oe->lowerstack[i].layer = &ofs->layers[i+1];
|-> }
|
| // 釋放臨時存放lower層路徑的path數(shù)組
|-> kfree(stack);
|-> return oe;

static struct dentry *
ovl_get_root(struct super_block *sb, struct dentry *upperdentry, struct ovl_entry *oe)
|-> struct dentry *root;
|-> struct ovl_path *lowerpath = &oe->lowerstack[0];
|-> struct ovl_inode_params oip = {
|->     .upperdentry = upperdentry,
|->     .lowerpath = lowerpath,
|-> };
|
|-> root = d_make_root(ovl_new_inode(sb, S_IFDIR, 0));
|-> root->d_fsdata = oe;
|
|-> ovl_inode_init(d_inode(root), &oip, ino, fsid);
|   |-> OVL_I(inode)->__upperdentry = oip->upperdentry;
|   |-> OVL_I(inode)->lower = igrab(d_inode(oip->lowerpath->dentry));
|
|-> ovl_dentry_update_reval(root, upperdentry, DCACHE_OP_WEAK_REVALIDATE);
|-> return root;