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Understanding the Linux Kernel, 3rd Edition by Marco Cesati, Daniel P. Bovet

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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 ...

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