Linux mount程式碼分析

1.    資料結構

vfsmount

struct vfsmount {
                struct list_head mnt_hash;          // hash表
                struct vfsmount*mnt_parent;   /* fs we are mounted on */
                struct dentry*mnt_mountpoint;              /* dentry ofmountpoint */
                struct dentry *mnt_root;              /* root of the mounted tree */|
                struct super_block*mnt_sb;      /* pointer to superblock */
                struct list_headmnt_mounts;    /* list of children,anchored here */
                struct list_headmnt_child;         /* and going throughtheir mnt_child */
                int mnt_flags;
                /* 4 bytes hole on 64bitsarches */
                const char *mnt_devname;         /* Name of device e.g. /dev/dsk/hda1 */
                struct list_head mnt_list;
                struct list_headmnt_expire;      /* link in fs-specificexpiry list */
                struct list_headmnt_share;        /* circular list ofshared mounts */
                struct list_headmnt_slave_list;/* list of slave mounts */
                struct list_headmnt_slave;         /* slave list entry */
                struct vfsmount*mnt_master;  /* slave is onmaster->mnt_slave_list */
                struct mnt_namespace*mnt_ns;             /* containingnamespace */
                int mnt_id;                                         /*mount identifier */
                int mnt_group_id;                           /* peer groupidentifier */
                /*
                 * We put mnt_count & mnt_expiry_mark atthe end of struct vfsmount
                 * to let these frequently modified fields in aseparate cache line
                 * (so that reads of mnt_flags wont ping-pongon SMP machines)
                 */
                atomic_t mnt_count;
                int mnt_expiry_mark;                    /* true if marked for expiry*/
                int mnt_pinned;
                int mnt_ghosts;
                int mnt_writers;
};

namespace

struct mnt_namespace {
                atomic_t                              count;
                struct vfsmount *            root;
                struct list_head                list;
                wait_queue_head_t poll;
                int event;
};

2.    初始化

void __init mnt_init(void)
                init_rwsem(&namespace_sem);
                static struct kmem_cache *mnt_cache =
                                kmem_cache_create("mnt_cache",sizeof(struct vfsmount),
                                                0, SLAB_HWCACHE_ALIGN |SLAB_PANIC, NULL);
                static struct list_head * mount_hashtable= (struct list_head *)__get_free_page(GFP_ATOMIC);
                for (u = 0; u <HASH_SIZE; u++)
                                INIT_LIST_HEAD(&mount_hashtable[u]);
                // 下面這行是初始化sysfs，放在這裡其實不太好，但由於初始化sysfs時用到了mount操作，而下面做其它檔案系統mount時又用到sysfs，所以也只能放這裡。
                sysfs_init();       
                fs_kobj =kobject_create_and_add("fs", NULL); //在sysfs中加上fs節點
                init_rootfs();
                init_mount_tree();

mnt_init最後調用了init_rootfs和init_mount_tree兩個函數，下面分別來看一下。

2.1 init_rootfs

int __init init_rootfs(void)
                int err;
                err =bdi_init(&ramfs_backing_dev_info);
                // 註冊rootfs檔案系統
                        err =register_filesystem(&rootfs_fs_type);

rootfs_fs_type定義在fs/ramfs/inode.c。

static struct file_system_type rootfs_fs_type = {
                .name                   = "rootfs",
                .get_sb                 = rootfs_get_sb,
                .kill_sb                 =kill_litter_super,
};

2.2 init_mount_tree

static void __init init_mount_tree(void)

                // 得到rootfs的vfsmount結構，設定裡面的超級塊等資訊，見下文

                structvfsmount *mnt =do_kern_mount("rootfs",0, "rootfs", NULL);

                // 根據mnt建立一個mnt_namespace，見下文

                structmnt_namespace *ns =create_mnt_ns(mnt);

                init_task.nsproxy->mnt_ns= ns;

                get_mnt_ns(ns);

                root.mnt= ns->root;

                root.dentry= ns->root->mnt_root;

                // 由於調init_mount_tree時init進程還沒啟動，下面的current宏由最開始時設定的棧指標決定，應該就是init_task的taskstruct結構體。

                set_fs_pwd(current->fs,&root);

                set_fs_root(current->fs,&root);

 

do_kern_mount的作用是分配一個vfsmount，同時裡面初始化了super block。

struct vfsmount * do_kern_mount(const char *fstype, intflags, const char *name, void *data)

                structfile_system_type *type = get_fs_type(fstype);

                structvfsmount *mnt;

                mnt =vfs_kern_mount(type, flags, name, data);

                put_filesystem(type);

                returnmnt;

struct vfsmount * vfs_kern_mount(struct file_system_type*type, int flags, const char *name, void *data)

                structvfsmount * mnt = alloc_vfsmnt(name);

                type->get_sb(type,flags, name, data, mnt);

                //mnt_mountpoint設定為mnt_root，即掛載點為自己

                mnt->mnt_mountpoint= mnt->mnt_root;

                // mnt_parent設成自己

                mnt->mnt_parent= mnt;

struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)

                structmnt_namespace *new_ns = alloc_mnt_ns();

                mnt->mnt_ns= new_ns;

                new_ns->root= mnt;

                list_add(&new_ns->list,&new_ns->root->mnt_list);

3.    檔案系統載入

mount操作包括remount,move_mount, mount塊裝置，mountloop裝置等，都由核心功能do_mount完成。本質上，mount操作的過程就是建立一個vfsmount結構，然後將此結構和掛載點關聯。關聯之後，目錄尋找時就能沿著vfsmount掛載點一級級向下尋找檔案了。

關於如何經過vfsmount尋找檔案，可以看函數kern_path。kern_path的作用為根據路徑得到dentry和inode結構，裡面會調用到path_walk，最終調用__follow_mount。

 

下面看一下mount塊裝置的實現。
long do_mount(char *dev_name, char *dir_name, char *type_page,
                                unsignedlong flags, void *data_page)

                struct path path;

                // 得到掛載點的vfsmount和dentry結構，儲存在structpath中。

                kern_path(dir_name,LOOKUP_FOLLOW, &path);

                。。。

                // 對塊裝置，調用到do_new_mount

                do_new_mount(&path, type_page, flags,mnt_flags, dev_name, data_page);

 

static int do_new_mount(struct path *path, char *type, int flags, int mnt_flags, char*name, void *data)

                struct vfsmount * mnt =do_kern_mount(type, flags, name, data);

                do_add_mount(mnt, path,mnt_flags, NULL);

 

int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flags, structlist_head *fslist)

    // 找到該目錄的當前掛載點的vfsmount和dentry結構

    while (d_mountpoint(path->dentry)&& follow_down(path))             ;

                // graft_tree主要作用是將新的vfsmount加到mounttree。

                graft_tree (newmnt, path);

 

loopmount與mount塊裝置類似，根據被載入裝置的superblock、dentry結構建立一個vfsmount結構，將該vfsmount掛到mounttree上。