kern_mount函数解析

1. kern_mount:

#define kern_mount(type) kern_mount_data(type, NULL)

    kern_mount相当于直接调用了kern_mount_data函数。

2.kern_mount_data

struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
{
    struct vfsmount *mnt;
    mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data); /**/
    if (!IS_ERR(mnt)) {
        /*
         * it is a longterm mount, don't release mnt until
         * we unmount before file sys is unregistered
        */
        real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL; /* 将包含vfsmount的mount对象的mnt_ns（struct mnt_namespace）赋值ERR_PTR(-EINVAL) */
    }
    return mnt;
}

3. vfs_kern_mount

struct vfsmount *
vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
{
    struct mount *mnt;
    struct dentry *root;

    if (!type)
        return ERR_PTR(-ENODEV);

    mnt = alloc_vfsmnt(name); /*分配一个mnt对象，并对其进行部分初始化*/
    if (!mnt)
        return ERR_PTR(-ENOMEM);

    if (flags & MS_KERNMOUNT)
        mnt->mnt.mnt_flags = MNT_INTERNAL;

    root = mount_fs(type, flags, name, data);  /* 获取该文件系统的根目录的dentry,同时也获取super_block */
    if (IS_ERR(root)) {
        free_vfsmnt(mnt);
        return ERR_CAST(root);
    }

    /* differentiate nand and emmc*/
    if(root->d_sb->s_mtd){
        g_rsrvd_size = NAND_DATA_RESERVED_SIZE;
    }
    /* 对mnt对象与root进行绑定 */
    mnt->mnt.mnt_root = root;
    mnt->mnt.mnt_sb = root->d_sb;
    mnt->mnt_mountpoint = mnt->mnt.mnt_root;
    mnt->mnt_parent = mnt;
    br_write_lock(&vfsmount_lock);
    list_add_tail(&mnt->mnt_instance, &root->d_sb->s_mounts);  /* 将mnt添加到root->d_sb->s_mounts链表中 */
    br_write_unlock(&vfsmount_lock);
    return &mnt->mnt;
}

4. mount_fs

struct dentry *
mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
{
    struct dentry *root;
    struct super_block *sb;
    char *secdata = NULL;
    int error = -ENOMEM;

    if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {  /* 在kern_mount调用中data为NULL，所以该if判断为假 */
        secdata = alloc_secdata();
        if (!secdata)
            goto out;

        error = security_sb_copy_data(data, secdata);
        if (error)
            goto out_free_secdata;
    }

    root = type->mount(type, flags, name, data);  /* 调用file_system_type中的 mount方法，通过该方法获取该文件系统的根目录dentry，同时也获取super_block. file_system_type的mount方法kernel提供以实现的函数：mount_single，mount_pseudo等 */
    if (IS_ERR(root)) {
        error = PTR_ERR(root);
        goto out_free_secdata;
    }
    sb = root->d_sb;
    BUG_ON(!sb);
    WARN_ON(!sb->s_bdi);
    WARN_ON(sb->s_bdi == &default_backing_dev_info);
    sb->s_flags |= MS_BORN;

    error = security_sb_kern_mount(sb, flags, secdata);
    if (error)
        goto out_sb;

    /*
     * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
     * but s_maxbytes was an unsigned long long for many releases. Throw
     * this warning for a little while to try and catch filesystems that
     * violate this rule.
     */
    WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
        "negative value (%lld)\n", type->name, sb->s_maxbytes);

    up_write(&sb->s_umount);
    free_secdata(secdata);
    return root;
out_sb:
    dput(root);
    deactivate_locked_super(sb);
out_free_secdata:
    free_secdata(secdata);
out:
    return ERR_PTR(error);
}

总结：kern_count函数主要用于那些没有实体介质的文件系统，该函数主要是获取文件系统的super_block对象与根目录的inode与dentry对象，并将这些对象加入到系统链表。