657 lines
24 KiB
Markdown
657 lines
24 KiB
Markdown
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# 34 | 块设备（上）：如何建立代理商销售模式？
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上一章，我们解析了文件系统，最后讲文件系统读写的流程到达底层的时候，没有更深入地分析下去，这是因为文件系统再往下就是硬盘设备了。上两节，我们解析了字符设备的mknod、打开和读写流程。那这一节我们就来讲块设备的mknod、打开流程，以及文件系统和下层的硬盘设备的读写流程。
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块设备一般会被格式化为文件系统，但是，下面的讲述中，你可能会有一点困惑。你会看到各种各样的dentry和inode。块设备涉及三种文件系统，所以你看到的这些dentry和inode可能都不是一回事儿，请注意分辨。
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块设备需要mknod吗？对于启动盘，你可能觉得，启动了就在那里了。可是如果我们要插进一块新的USB盘，还是要有这个操作的。
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mknod还是会创建在/dev路径下面，这一点和字符设备一样。/dev路径下面是devtmpfs文件系统。**这是块设备遇到的第一个文件系统**。我们会为这个块设备文件，分配一个特殊的inode，这一点和字符设备也是一样的。只不过字符设备走S\_ISCHR这个分支，对应inode的file\_operations是def\_chr\_fops；而块设备走S\_ISBLK这个分支，对应的inode的file\_operations是def\_blk\_fops。这里要注意，inode里面的i\_rdev被设置成了块设备的设备号dev\_t，这个我们后面会用到，你先记住有这么一回事儿。
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```
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void init_special_inode(struct inode *inode, umode_t mode, dev_t rdev)
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{
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inode->i_mode = mode;
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if (S_ISCHR(mode)) {
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inode->i_fop = &def_chr_fops;
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inode->i_rdev = rdev;
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} else if (S_ISBLK(mode)) {
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inode->i_fop = &def_blk_fops;
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inode->i_rdev = rdev;
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} else if (S_ISFIFO(mode))
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inode->i_fop = &pipefifo_fops;
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else if (S_ISSOCK(mode))
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; /* leave it no_open_fops */
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}
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```
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特殊inode的默认file\_operations是def\_blk\_fops，就像字符设备一样，有打开、读写这个块设备文件，但是我们常规操作不会这样做。我们会将这个块设备文件mount到一个文件夹下面。
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```
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const struct file_operations def_blk_fops = {
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.open = blkdev_open,
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.release = blkdev_close,
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.llseek = block_llseek,
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.read_iter = blkdev_read_iter,
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.write_iter = blkdev_write_iter,
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.mmap = generic_file_mmap,
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.fsync = blkdev_fsync,
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.unlocked_ioctl = block_ioctl,
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.splice_read = generic_file_splice_read,
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.splice_write = iter_file_splice_write,
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.fallocate = blkdev_fallocate,
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};
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```
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不过，这里我们还是简单看一下，打开这个块设备的操作blkdev\_open。它里面调用的是blkdev\_get打开这个块设备，了解到这一点就可以了。
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接下来，我们要调用mount，将这个块设备文件挂载到一个文件夹下面。如果这个块设备原来被格式化为一种文件系统的格式，例如ext4，那我们调用的就是ext4相应的mount操作。**这是块设备遇到的第二个文件系统**，也是向这个块设备读写文件，需要基于的主流文件系统。咱们在文件系统那一节解析的对于文件的读写流程，都是基于这个文件系统的。
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还记得，咱们注册ext4文件系统的时候，有下面这样的结构：
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```
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static struct file_system_type ext4_fs_type = {
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.owner = THIS_MODULE,
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.name = "ext4",
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.mount = ext4_mount,
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.kill_sb = kill_block_super,
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.fs_flags = FS_REQUIRES_DEV,
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};
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```
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在将一个硬盘的块设备mount成为ext4的时候，我们会调用ext4\_mount->mount\_bdev。
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```
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static struct dentry *ext4_mount(struct file_system_type *fs_type, int flags, const char *dev_name, void *data)
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{
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return mount_bdev(fs_type, flags, dev_name, data, ext4_fill_super);
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}
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struct dentry *mount_bdev(struct file_system_type *fs_type,
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int flags, const char *dev_name, void *data,
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int (*fill_super)(struct super_block *, void *, int))
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{
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struct block_device *bdev;
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struct super_block *s;
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fmode_t mode = FMODE_READ | FMODE_EXCL;
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int error = 0;
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if (!(flags & MS_RDONLY))
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mode |= FMODE_WRITE;
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bdev = blkdev_get_by_path(dev_name, mode, fs_type);
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......
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s = sget(fs_type, test_bdev_super, set_bdev_super, flags | MS_NOSEC, bdev);
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......
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return dget(s->s_root);
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......
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}
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```
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mount\_bdev主要做了两件大事情。第一，blkdev\_get\_by\_path根据/dev/xxx这个名字，找到相应的设备并打开它；第二，sget根据打开的设备文件，填充ext4文件系统的super\_block，从而以此为基础，建立一整套咱们在文件系统那一章讲的体系。
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一旦这套体系建立起来以后，对于文件的读写都是通过ext4文件系统这个体系进行的，创建的inode结构也是指向ext4文件系统的。文件系统那一章我们只解析了这部分，由于没有到达底层，也就没有关注块设备相关的操作。这一章我们重新回过头来，一方面看mount的时候，对于块设备都做了哪些操作，另一方面看读写的时候，到了底层，对于块设备做了哪些操作。
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这里我们先来看mount\_bdev做的第一件大事情，通过blkdev\_get\_by\_path，根据设备名/dev/xxx，得到struct block\_device \*bdev。
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```
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/**
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* blkdev_get_by_path - open a block device by name
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* @path: path to the block device to open
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* @mode: FMODE_* mask
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* @holder: exclusive holder identifier
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*
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* Open the blockdevice described by the device file at @path. @mode
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* and @holder are identical to blkdev_get().
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*
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* On success, the returned block_device has reference count of one.
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*/
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struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
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void *holder)
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{
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struct block_device *bdev;
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int err;
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bdev = lookup_bdev(path);
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......
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err = blkdev_get(bdev, mode, holder);
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......
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return bdev;
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}
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```
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blkdev\_get\_by\_path干了两件事情。第一个，lookup\_bdev根据设备路径/dev/xxx得到block\_device。第二个，打开这个设备，调用blkdev\_get。
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咱们上面分析过def\_blk\_fops的默认打开设备函数blkdev\_open，它也是调用blkdev\_get的。块设备的打开往往不是直接调用设备文件的打开函数，而是调用mount来打开的。
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```
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/**
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* lookup_bdev - lookup a struct block_device by name
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* @pathname: special file representing the block device
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*
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* Get a reference to the blockdevice at @pathname in the current
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* namespace if possible and return it. Return ERR_PTR(error)
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* otherwise.
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*/
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struct block_device *lookup_bdev(const char *pathname)
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{
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struct block_device *bdev;
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struct inode *inode;
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struct path path;
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int error;
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if (!pathname || !*pathname)
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return ERR_PTR(-EINVAL);
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error = kern_path(pathname, LOOKUP_FOLLOW, &path);
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if (error)
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return ERR_PTR(error);
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inode = d_backing_inode(path.dentry);
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......
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bdev = bd_acquire(inode);
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......
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goto out;
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}
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```
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lookup\_bdev这里的pathname是设备的文件名，例如/dev/xxx。这个文件是在devtmpfs文件系统中的，kern\_path可以在这个文件系统里面，一直找到它对应的dentry。接下来，d\_backing\_inode会获得inode。这个inode就是那个init\_special\_inode生成的特殊inode。
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接下来，bd\_acquire通过这个特殊的inode，找到struct block\_device。
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```
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static struct block_device *bd_acquire(struct inode *inode)
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{
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struct block_device *bdev;
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......
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bdev = bdget(inode->i_rdev);
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if (bdev) {
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spin_lock(&bdev_lock);
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if (!inode->i_bdev) {
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/*
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* We take an additional reference to bd_inode,
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* and it's released in clear_inode() of inode.
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* So, we can access it via ->i_mapping always
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* without igrab().
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*/
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bdgrab(bdev);
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inode->i_bdev = bdev;
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inode->i_mapping = bdev->bd_inode->i_mapping;
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}
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}
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return bdev;
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}
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```
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bd\_acquire中最主要的就是调用bdget，它的参数是特殊inode的i\_rdev。这里面在mknod的时候，放的是设备号dev\_t。
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```
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struct block_device *bdget(dev_t dev)
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{
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struct block_device *bdev;
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struct inode *inode;
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inode = iget5_locked(blockdev_superblock, hash(dev),
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bdev_test, bdev_set, &dev);
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bdev = &BDEV_I(inode)->bdev;
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if (inode->i_state & I_NEW) {
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bdev->bd_contains = NULL;
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bdev->bd_super = NULL;
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bdev->bd_inode = inode;
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bdev->bd_block_size = i_blocksize(inode);
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bdev->bd_part_count = 0;
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bdev->bd_invalidated = 0;
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inode->i_mode = S_IFBLK;
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inode->i_rdev = dev;
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inode->i_bdev = bdev;
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inode->i_data.a_ops = &def_blk_aops;
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mapping_set_gfp_mask(&inode->i_data, GFP_USER);
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spin_lock(&bdev_lock);
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list_add(&bdev->bd_list, &all_bdevs);
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spin_unlock(&bdev_lock);
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unlock_new_inode(inode);
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}
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return bdev;
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}
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```
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**在bdget中，我们遇到了第三个文件系统，bdev伪文件系统**。bdget函数根据传进来的dev\_t，在blockdev\_superblock这个文件系统里面找到inode。这里注意，这个inode已经不是devtmpfs文件系统的inode了。blockdev\_superblock的初始化在整个系统初始化的时候，会调用bdev\_cache\_init进行初始化。它的定义如下：
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```
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struct super_block *blockdev_superblock __read_mostly;
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static struct file_system_type bd_type = {
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.name = "bdev",
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.mount = bd_mount,
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.kill_sb = kill_anon_super,
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};
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void __init bdev_cache_init(void)
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{
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int err;
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static struct vfsmount *bd_mnt;
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bdev_cachep = kmem_cache_create("bdev_cache", sizeof(struct bdev_inode), 0, (SLAB_HWCACHE_ALIGN|SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD|SLAB_ACCOUNT|SLAB_PANIC), init_once);
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err = register_filesystem(&bd_type);
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if (err)
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panic("Cannot register bdev pseudo-fs");
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bd_mnt = kern_mount(&bd_type);
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if (IS_ERR(bd_mnt))
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panic("Cannot create bdev pseudo-fs");
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blockdev_superblock = bd_mnt->mnt_sb; /* For writeback */
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}
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```
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所有表示块设备的inode都保存在伪文件系统 bdev中，这些对用户层不可见，主要为了方便块设备的管理。Linux将块设备的block\_device和bdev文件系统的块设备的inode，通过struct bdev\_inode进行关联。所以，在bdget中，BDEV\_I就是通过bdev文件系统的inode，获得整个struct bdev\_inode结构的地址，然后取成员bdev，得到block\_device。
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```
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struct bdev_inode {
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struct block_device bdev;
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struct inode vfs_inode;
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};
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```
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绕了一大圈，我们终于通过设备文件/dev/xxx，获得了设备的结构block\_device。有点儿绕，我们再捋一下。设备文件/dev/xxx在devtmpfs文件系统中，找到devtmpfs文件系统中的inode，里面有dev\_t。我们可以通过dev\_t，在伪文件系统 bdev中找到对应的inode，然后根据struct bdev\_inode找到关联的block\_device。
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接下来，blkdev\_get\_by\_path开始做第二件事情，在找到block\_device之后，要调用blkdev\_get打开这个设备。blkdev\_get会调用\_\_blkdev\_get。
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在分析打开一个设备之前，我们先来看block\_device这个结构是什么样的。
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```
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struct block_device {
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dev_t bd_dev; /* not a kdev_t - it's a search key */
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int bd_openers;
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struct super_block * bd_super;
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......
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struct block_device * bd_contains;
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unsigned bd_block_size;
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struct hd_struct * bd_part;
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|||
|
|
unsigned bd_part_count;
|
|||
|
|
int bd_invalidated;
|
|||
|
|
struct gendisk * bd_disk;
|
|||
|
|
struct request_queue * bd_queue;
|
|||
|
|
struct backing_dev_info *bd_bdi;
|
|||
|
|
struct list_head bd_list;
|
|||
|
|
......
|
|||
|
|
} ;
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
你应该能发现，这个结构和其他几个结构有着千丝万缕的联系，比较复杂。这是因为块设备本身就比较复杂。
|
|||
|
|
|
|||
|
|
比方说，我们有一个磁盘/dev/sda，我们既可以把它整个格式化成一个文件系统，也可以把它分成多个分区/dev/sda1、 /dev/sda2，然后把每个分区格式化成不同的文件系统。如果我们访问某个分区的设备文件/dev/sda2，我们应该能知道它是哪个磁盘设备的。按说它们的驱动应该是一样的。如果我们访问整个磁盘的设备文件/dev/sda，我们也应该能知道它分了几个区域，所以就有了下图这个复杂的关系结构。
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
struct gendisk是用来描述整个设备的，因而上面的例子中，gendisk只有一个实例，指向/dev/sda。它的定义如下：
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
struct gendisk {
|
|||
|
|
int major; /* major number of driver */
|
|||
|
|
int first_minor;
|
|||
|
|
int minors; /* maximum number of minors, =1 for disks that can't be partitioned. */
|
|||
|
|
char disk_name[DISK_NAME_LEN]; /* name of major driver */
|
|||
|
|
char *(*devnode)(struct gendisk *gd, umode_t *mode);
|
|||
|
|
......
|
|||
|
|
struct disk_part_tbl __rcu *part_tbl;
|
|||
|
|
struct hd_struct part0;
|
|||
|
|
|
|||
|
|
|
|||
|
|
const struct block_device_operations *fops;
|
|||
|
|
struct request_queue *queue;
|
|||
|
|
void *private_data;
|
|||
|
|
|
|||
|
|
|
|||
|
|
int flags;
|
|||
|
|
struct kobject *slave_dir;
|
|||
|
|
......
|
|||
|
|
};
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
这里major是主设备号，first\_minor表示第一个分区的从设备号，minors表示分区的数目。
|
|||
|
|
|
|||
|
|
disk\_name给出了磁盘块设备的名称。
|
|||
|
|
|
|||
|
|
struct disk\_part\_tbl结构里是一个struct hd\_struct的数组，用于表示各个分区。struct block\_device\_operations fops指向对于这个块设备的各种操作。struct request\_queue queue是表示在这个块设备上的请求队列。
|
|||
|
|
|
|||
|
|
struct hd\_struct是用来表示某个分区的，在上面的例子中，有两个hd\_struct的实例，分别指向/dev/sda1、 /dev/sda2。它的定义如下：
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
struct hd_struct {
|
|||
|
|
sector_t start_sect;
|
|||
|
|
sector_t nr_sects;
|
|||
|
|
......
|
|||
|
|
struct device __dev;
|
|||
|
|
struct kobject *holder_dir;
|
|||
|
|
int policy, partno;
|
|||
|
|
struct partition_meta_info *info;
|
|||
|
|
......
|
|||
|
|
struct disk_stats dkstats;
|
|||
|
|
struct percpu_ref ref;
|
|||
|
|
struct rcu_head rcu_head;
|
|||
|
|
};
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
在hd\_struct中，比较重要的成员变量保存了如下的信息：从磁盘的哪个扇区开始，到哪个扇区结束。
|
|||
|
|
|
|||
|
|
而block\_device既可以表示整个块设备，也可以表示某个分区，所以对于上面的例子，block\_device有三个实例，分别指向/dev/sda1、/dev/sda2、/dev/sda。
|
|||
|
|
|
|||
|
|
block\_device的成员变量bd\_disk，指向的gendisk就是整个块设备。这三个实例都指向同一个gendisk。bd\_part指向的某个分区的hd\_struct，bd\_contains指向的是整个块设备的block\_device。
|
|||
|
|
|
|||
|
|
了解了这些复杂的关系，我们再来看打开设备文件的代码，就会清晰很多。
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
|
|||
|
|
{
|
|||
|
|
struct gendisk *disk;
|
|||
|
|
struct module *owner;
|
|||
|
|
int ret;
|
|||
|
|
int partno;
|
|||
|
|
int perm = 0;
|
|||
|
|
|
|||
|
|
|
|||
|
|
if (mode & FMODE_READ)
|
|||
|
|
perm |= MAY_READ;
|
|||
|
|
if (mode & FMODE_WRITE)
|
|||
|
|
perm |= MAY_WRITE;
|
|||
|
|
......
|
|||
|
|
disk = get_gendisk(bdev->bd_dev, &partno);
|
|||
|
|
......
|
|||
|
|
owner = disk->fops->owner;
|
|||
|
|
......
|
|||
|
|
if (!bdev->bd_openers) {
|
|||
|
|
bdev->bd_disk = disk;
|
|||
|
|
bdev->bd_queue = disk->queue;
|
|||
|
|
bdev->bd_contains = bdev;
|
|||
|
|
|
|||
|
|
|
|||
|
|
if (!partno) {
|
|||
|
|
ret = -ENXIO;
|
|||
|
|
bdev->bd_part = disk_get_part(disk, partno);
|
|||
|
|
......
|
|||
|
|
if (disk->fops->open) {
|
|||
|
|
ret = disk->fops->open(bdev, mode);
|
|||
|
|
......
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
|
|||
|
|
if (!ret)
|
|||
|
|
bd_set_size(bdev,(loff_t)get_capacity(disk)<<9);
|
|||
|
|
|
|||
|
|
|
|||
|
|
if (bdev->bd_invalidated) {
|
|||
|
|
if (!ret)
|
|||
|
|
rescan_partitions(disk, bdev);
|
|||
|
|
......
|
|||
|
|
}
|
|||
|
|
......
|
|||
|
|
} else {
|
|||
|
|
struct block_device *whole;
|
|||
|
|
whole = bdget_disk(disk, 0);
|
|||
|
|
......
|
|||
|
|
ret = __blkdev_get(whole, mode, 1);
|
|||
|
|
......
|
|||
|
|
bdev->bd_contains = whole;
|
|||
|
|
bdev->bd_part = disk_get_part(disk, partno);
|
|||
|
|
......
|
|||
|
|
bd_set_size(bdev, (loff_t)bdev->bd_part->nr_sects << 9);
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
......
|
|||
|
|
bdev->bd_openers++;
|
|||
|
|
if (for_part)
|
|||
|
|
bdev->bd_part_count++;
|
|||
|
|
.....
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
在\_\_blkdev\_get函数中，我们先调用get\_gendisk，根据block\_device获取gendisk。具体代码如下：
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
/**
|
|||
|
|
* get_gendisk - get partitioning information for a given device
|
|||
|
|
* @devt: device to get partitioning information for
|
|||
|
|
* @partno: returned partition index
|
|||
|
|
*
|
|||
|
|
* This function gets the structure containing partitioning
|
|||
|
|
* information for the given device @devt.
|
|||
|
|
*/
|
|||
|
|
struct gendisk *get_gendisk(dev_t devt, int *partno)
|
|||
|
|
{
|
|||
|
|
struct gendisk *disk = NULL;
|
|||
|
|
|
|||
|
|
|
|||
|
|
if (MAJOR(devt) != BLOCK_EXT_MAJOR) {
|
|||
|
|
struct kobject *kobj;
|
|||
|
|
|
|||
|
|
|
|||
|
|
kobj = kobj_lookup(bdev_map, devt, partno);
|
|||
|
|
if (kobj)
|
|||
|
|
disk = dev_to_disk(kobj_to_dev(kobj));
|
|||
|
|
} else {
|
|||
|
|
struct hd_struct *part;
|
|||
|
|
part = idr_find(&ext_devt_idr, blk_mangle_minor(MINOR(devt)));
|
|||
|
|
if (part && get_disk(part_to_disk(part))) {
|
|||
|
|
*partno = part->partno;
|
|||
|
|
disk = part_to_disk(part);
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
return disk;
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
我们可以想象这里面有两种情况。第一种情况是，block\_device是指向整个磁盘设备的。这个时候，我们只需要根据dev\_t，在bdev\_map中将对应的gendisk拿出来就好。
|
|||
|
|
|
|||
|
|
bdev\_map是干什么的呢？前面咱们学习字符设备驱动的时候讲过，任何一个字符设备初始化的时候，都需要调用\_\_register\_chrdev\_region，注册这个字符设备。对于块设备也是类似的，每一个块设备驱动初始化的时候，都会调用add\_disk注册一个gendisk。
|
|||
|
|
|
|||
|
|
这里需要说明一下，gen的意思是general通用的意思，也就是说，所有的块设备，不仅仅是硬盘disk，都会用一个gendisk来表示，然后通过调用链add\_disk->device\_add\_disk->blk\_register\_region，将dev\_t和一个gendisk关联起来，保存在bdev\_map中。
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
static struct kobj_map *bdev_map;
|
|||
|
|
|
|||
|
|
|
|||
|
|
static inline void add_disk(struct gendisk *disk)
|
|||
|
|
{
|
|||
|
|
device_add_disk(NULL, disk);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
|
|||
|
|
/**
|
|||
|
|
* device_add_disk - add partitioning information to kernel list
|
|||
|
|
* @parent: parent device for the disk
|
|||
|
|
* @disk: per-device partitioning information
|
|||
|
|
*
|
|||
|
|
* This function registers the partitioning information in @disk
|
|||
|
|
* with the kernel.
|
|||
|
|
*/
|
|||
|
|
void device_add_disk(struct device *parent, struct gendisk *disk)
|
|||
|
|
{
|
|||
|
|
......
|
|||
|
|
blk_register_region(disk_devt(disk), disk->minors, NULL,
|
|||
|
|
exact_match, exact_lock, disk);
|
|||
|
|
.....
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
|
|||
|
|
/*
|
|||
|
|
* Register device numbers dev..(dev+range-1)
|
|||
|
|
* range must be nonzero
|
|||
|
|
* The hash chain is sorted on range, so that subranges can override.
|
|||
|
|
*/
|
|||
|
|
void blk_register_region(dev_t devt, unsigned long range, struct module *module,
|
|||
|
|
struct kobject *(*probe)(dev_t, int *, void *),
|
|||
|
|
int (*lock)(dev_t, void *), void *data)
|
|||
|
|
{
|
|||
|
|
kobj_map(bdev_map, devt, range, module, probe, lock, data);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
get\_gendisk要处理的第二种情况是，block\_device是指向某个分区的。这个时候我们要先得到hd\_struct，然后通过hd\_struct，找到对应的整个设备的gendisk，并且把partno设置为分区号。
|
|||
|
|
|
|||
|
|
我们再回到\_\_blkdev\_get函数中，得到gendisk。接下来我们可以分两种情况。
|
|||
|
|
|
|||
|
|
如果partno为0，也就是说，打开的是整个设备而不是分区，那我们就调用disk\_get\_part，获取gendisk中的分区数组，然后调用block\_device\_operations里面的open函数打开设备。
|
|||
|
|
|
|||
|
|
如果partno不为0，也就是说打开的是分区，那我们就获取整个设备的block\_device，赋值给变量struct block\_device \*whole，然后调用递归\_\_blkdev\_get，打开whole代表的整个设备，将bd\_contains设置为变量whole。
|
|||
|
|
|
|||
|
|
block\_device\_operations就是在驱动层了。例如在drivers/scsi/sd.c里面，也就是MODULE\_DESCRIPTION(“SCSI disk (sd) driver”)中，就有这样的定义。
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
static const struct block_device_operations sd_fops = {
|
|||
|
|
.owner = THIS_MODULE,
|
|||
|
|
.open = sd_open,
|
|||
|
|
.release = sd_release,
|
|||
|
|
.ioctl = sd_ioctl,
|
|||
|
|
.getgeo = sd_getgeo,
|
|||
|
|
#ifdef CONFIG_COMPAT
|
|||
|
|
.compat_ioctl = sd_compat_ioctl,
|
|||
|
|
#endif
|
|||
|
|
.check_events = sd_check_events,
|
|||
|
|
.revalidate_disk = sd_revalidate_disk,
|
|||
|
|
.unlock_native_capacity = sd_unlock_native_capacity,
|
|||
|
|
.pr_ops = &sd_pr_ops,
|
|||
|
|
};
|
|||
|
|
|
|||
|
|
|
|||
|
|
/**
|
|||
|
|
* sd_open - open a scsi disk device
|
|||
|
|
* @bdev: Block device of the scsi disk to open
|
|||
|
|
* @mode: FMODE_* mask
|
|||
|
|
*
|
|||
|
|
* Returns 0 if successful. Returns a negated errno value in case
|
|||
|
|
* of error.
|
|||
|
|
**/
|
|||
|
|
static int sd_open(struct block_device *bdev, fmode_t mode)
|
|||
|
|
{
|
|||
|
|
......
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
在驱动层打开了磁盘设备之后，我们可以看到，在这个过程中，block\_device相应的成员变量该填的都填上了，这才完成了mount\_bdev的第一件大事，通过blkdev\_get\_by\_path得到block\_device。
|
|||
|
|
|
|||
|
|
接下来就是第二件大事情，我们要通过sget，将block\_device塞进superblock里面。注意，调用sget的时候，有一个参数是一个函数set\_bdev\_super。这里面将block\_device设置进了super\_block。而sget要做的，就是分配一个super\_block，然后调用set\_bdev\_super这个callback函数。这里的super\_block是ext4文件系统的super\_block。
|
|||
|
|
|
|||
|
|
sget(fs\_type, test\_bdev\_super, set\_bdev\_super, flags | MS\_NOSEC, bdev);
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
static int set_bdev_super(struct super_block *s, void *data)
|
|||
|
|
{
|
|||
|
|
s->s_bdev = data;
|
|||
|
|
s->s_dev = s->s_bdev->bd_dev;
|
|||
|
|
s->s_bdi = bdi_get(s->s_bdev->bd_bdi);
|
|||
|
|
return 0;
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
|
|||
|
|
/**
|
|||
|
|
* sget - find or create a superblock
|
|||
|
|
* @type: filesystem type superblock should belong to
|
|||
|
|
* @test: comparison callback
|
|||
|
|
* @set: setup callback
|
|||
|
|
* @flags: mount flags
|
|||
|
|
* @data: argument to each of them
|
|||
|
|
*/
|
|||
|
|
struct super_block *sget(struct file_system_type *type,
|
|||
|
|
int (*test)(struct super_block *,void *),
|
|||
|
|
int (*set)(struct super_block *,void *),
|
|||
|
|
int flags,
|
|||
|
|
void *data)
|
|||
|
|
{
|
|||
|
|
......
|
|||
|
|
return sget_userns(type, test, set, flags, user_ns, data);
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
|
|||
|
|
/**
|
|||
|
|
* sget_userns - find or create a superblock
|
|||
|
|
* @type: filesystem type superblock should belong to
|
|||
|
|
* @test: comparison callback
|
|||
|
|
* @set: setup callback
|
|||
|
|
* @flags: mount flags
|
|||
|
|
* @user_ns: User namespace for the super_block
|
|||
|
|
* @data: argument to each of them
|
|||
|
|
*/
|
|||
|
|
struct super_block *sget_userns(struct file_system_type *type,
|
|||
|
|
int (*test)(struct super_block *,void *),
|
|||
|
|
int (*set)(struct super_block *,void *),
|
|||
|
|
int flags, struct user_namespace *user_ns,
|
|||
|
|
void *data)
|
|||
|
|
{
|
|||
|
|
struct super_block *s = NULL;
|
|||
|
|
struct super_block *old;
|
|||
|
|
int err;
|
|||
|
|
......
|
|||
|
|
if (!s) {
|
|||
|
|
s = alloc_super(type, (flags & ~MS_SUBMOUNT), user_ns);
|
|||
|
|
......
|
|||
|
|
}
|
|||
|
|
err = set(s, data);
|
|||
|
|
......
|
|||
|
|
s->s_type = type;
|
|||
|
|
strlcpy(s->s_id, type->name, sizeof(s->s_id));
|
|||
|
|
list_add_tail(&s->s_list, &super_blocks);
|
|||
|
|
hlist_add_head(&s->s_instances, &type->fs_supers);
|
|||
|
|
spin_unlock(&sb_lock);
|
|||
|
|
get_filesystem(type);
|
|||
|
|
register_shrinker(&s->s_shrink);
|
|||
|
|
return s;
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
好了，到此为止，mount中一个块设备的过程就结束了。设备打开了，形成了block\_device结构，并且塞到了super\_block中。
|
|||
|
|
|
|||
|
|
有了ext4文件系统的super\_block之后，接下来对于文件的读写过程，就和文件系统那一章的过程一摸一样了。只要不涉及真正写入设备的代码，super\_block中的这个block\_device就没啥用处。这也是为什么文件系统那一章，我们丝毫感觉不到它的存在，但是一旦到了底层，就到了block\_device起作用的时候了，这个我们下一节仔细分析。
|
|||
|
|
|
|||
|
|
## 总结时刻
|
|||
|
|
|
|||
|
|
从这一节我们可以看出，块设备比字符设备复杂多了，涉及三个文件系统，工作过程我用一张图总结了一下，下面带你总结一下。
|
|||
|
|
|
|||
|
|
1. 所有的块设备被一个map结构管理从dev\_t到gendisk的映射；
|
|||
|
|
2. 所有的block\_device表示的设备或者分区都在bdev文件系统的inode列表中；
|
|||
|
|
3. mknod创建出来的块设备文件在devtemfs文件系统里面，特殊inode里面有块设备号；
|
|||
|
|
4. mount一个块设备上的文件系统，调用这个文件系统的mount接口；
|
|||
|
|
5. 通过按照/dev/xxx在文件系统devtmpfs文件系统上搜索到特殊inode，得到块设备号；
|
|||
|
|
6. 根据特殊inode里面的dev\_t在bdev文件系统里面找到inode；
|
|||
|
|
7. 根据bdev文件系统上的inode找到对应的block\_device，根据dev\_t在map中找到gendisk，将两者关联起来；
|
|||
|
|
8. 找到block\_device后打开设备，调用和block\_device关联的gendisk里面的block\_device\_operations打开设备；
|
|||
|
|
9. 创建被mount的文件系统的super\_block。
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
## 课堂练习
|
|||
|
|
|
|||
|
|
到这里，你是否真的体会到了Linux里面“一切皆文件”了呢？那个特殊的inode除了能够表示字符设备和块设备，还能表示什么呢？请你看代码分析一下。
|
|||
|
|
|
|||
|
|
欢迎留言和我分享你的疑惑和见解 ，也欢迎可以收藏本节内容，反复研读。你也可以把今天的内容分享给你的朋友，和他一起学习和进步。
|
|||
|
|
|