塊設備的分區信息由struct hd_struct結構描述，其中最重要的信息就是分區的起始扇區號和分區的大小。所有分區信息都一起保存在gendisk的part_tbl結構中，同時每個分區的block_device也可以通過bd_part來查詢對應的分區信息。

下圖描述了block_device,gendisk以及分區描述符之間的關系(塊設備有兩個分區)

下面通過打開一個塊設備的過程，來理解這些結構之間的聯系。

對于塊設備文件的操作，通過block_dev偽文件系統來完成，open操作定義的函數為blkdev_open()

blkdev_open的主要任務有兩個

1.獲取設備的block_device信息

2.從gendisk中讀取相關信息保存到block_device，同時建立數據結構之間的聯系

static int blkdev_open(struct inode * inode, struct file * filp)

{

struct block_device *bdev;

int res;

/*

* Preserve backwards compatibility and allow large file access

* even if userspace doesn't ask for it explicitly. Some mkfs

* binary needs it. We might want to drop this workaround

* during an unstable branch.

*/

filp->f_flags |= O_LARGEFILE;

if (filp->f_flags & O_NDELAY)

filp->f_mode |= FMODE_NDELAY;

if (filp->f_flags & O_EXCL)

filp->f_mode |= FMODE_EXCL;

if ((filp->f_flags & O_ACCMODE) == 3)

filp->f_mode |= FMODE_WRITE_IOCTL;

bdev = bd_acquire(inode);//獲取block device實例

if (bdev == NULL)

return -ENOMEM;

filp->f_mapping = bdev->bd_inode->i_mapping;

res = blkdev_get(bdev, filp->f_mode);//通過gendisk獲取信息并建立聯系

if (res)

return res;

if (filp->f_mode & FMODE_EXCL) {

res = bd_claim(bdev, filp);

if (res)

goto out_blkdev_put;

}

return 0;

out_blkdev_put:

blkdev_put(bdev, filp->f_mode);

return res;

}

bd_acquire()負責獲取block_device的實例

static struct block_device *bd_acquire(struct inode *inode)

{

struct block_device *bdev;

spin_lock(&bdev_lock);

bdev = inode->i_bdev;//如果這個設備之前被打開過則可以直接通過i_bdev獲取

if (bdev) {

atomic_inc(&bdev->bd_inode->i_count);

spin_unlock(&bdev_lock);

return bdev;

}

spin_unlock(&bdev_lock);

bdev = bdget(inode->i_rdev);//通過設備號的信息來獲取block device實例

if (bdev) {

spin_lock(&bdev_lock);

if (!inode->i_bdev) {

/*

* We take an additional bd_inode->i_count for inode,

* and it's released in clear_inode() of inode.

* So, we can access it via ->i_mapping always

* without igrab().

*/

atomic_inc(&bdev->bd_inode->i_count);

inode->i_bdev = bdev;

inode->i_mapping = bdev->bd_inode->i_mapping;

list_add(&inode->i_devices, &bdev->bd_inodes);

}

spin_unlock(&bdev_lock);

}

return bdev;

}

-------------------------------分割線-------------------------------

struct block_device *bdget(dev_t dev)

{

struct block_device *bdev;

struct inode *inode;

/*這里先在inode的哈希表中進行查找與dev設備號對應的inode，如果沒找到的話，

則通過bdev偽文件系統創建bdev_inode(包含inode和block device的結構體)*/

inode = iget5_locked(blockdev_superblock, hash(dev),

bdev_test, bdev_set, &dev);

if (!inode)

return NULL;

//通過inode獲取bdev_inode,再通過bdev_inode獲取block device實例

bdev = &BDEV_I(inode)->bdev;

if (inode->i_state & I_NEW) {

/*分別設置block device和inode的相關域*/

bdev->bd_contains = NULL;

bdev->bd_inode = inode;

bdev->bd_block_size = (1 << inode->i_blkbits);

bdev->bd_part_count = 0;

bdev->bd_invalidated = 0;

inode->i_mode = S_IFBLK;

inode->i_rdev = dev;

inode->i_bdev = bdev;

inode->i_data.a_ops = &def_blk_aops;

mapping_set_gfp_mask(&inode->i_data, GFP_USER);

inode->i_data.backing_dev_info = &default_backing_dev_info;

spin_lock(&bdev_lock);

list_add(&bdev->bd_list, &all_bdevs);

spin_unlock(&bdev_lock);

unlock_new_inode(inode);

}

return bdev;

}