Understanding the Linux Kernel, 3rd Edition

Opening a Block Device File

We conclude this chapter by describing the steps performed by the VFS when opening a block device file.

The kernel opens a block device file every time that a filesystem is mounted over a disk or partition, every time that a swap partition is activated, and every time that a User Mode process issues an open( ) system call on a block device file. In all cases, the kernel executes essentially the same operations: it looks for the block device descriptor (possibly allocating a new descriptor if the block device is not already in use), and sets up the file operation methods for the forthcoming data transfers.

In the section "VFS Handling of Device Files" in Chapter 13, we described how the dentry_open( ) function customizes the methods of the file object when a device file is opened. In this case, the f_op field of the file object is set to the address of the def_blk_fops table, whose content is shown in Table 14-10.

Table 14-10. The default block device file operations (def_blk_fops table)

Method	Function
open	blkdev_open( )
release	blkdev_close( )
llseek	block_llseek( )
read	generic_file_read( )
write	blkdev_file_write( )
aio_read	generic_file_aio_read( )
aio_write	blkdev_file_aio_write( )
mmap	generic_file_mmap( )
fsync	block_fsync( )
ioctl	block_ioctl( )
compat-ioctl	compat_blkdev_ioctl( )
readv	generic_file_readv( )
writev	generic_file_write_nolock( )
sendfile	generic_file_sendfile( )

Here we are only concerned ...