Creating Block Devices

In the world of Linux kernel development, block devices play a critical role in how the operating system interacts with hardware storage devices like hard drives, SSDs, and USB sticks. Unlike character devices, which handle data as a stream of bytes, block devices manage data in fixed-size blocks, allowing for efficient reading and writing of large amounts of data at once. This article is going to focus on how to implement block devices in your kernel modules and give you a clearer understanding of their functioning.

Understanding Block Devices

Before diving into the actual implementation, let's quickly recap what block devices are. Block devices allow the kernel to read from and write to data storage in a way that is both efficient and manageable. They provide a buffer for data that can be accessed in blocks rather than byte by byte. This access pattern is crucial for operations like file system management.

In Linux, block devices are represented by the struct block_device data structure, which contains several fields relevant to the state and management of the device. Understanding how to interact with these structures will be key to your success as we move forward.

Requirements for Creating a Block Device

Kernel Development Environment: You must have a working environment set up to compile and test your kernel modules. This generally includes having the kernel headers and build tools installed.
Basic Knowledge of Kernel Programming: Familiarity with writing kernel modules and handling data structures in C will go a long way.
Debugging Tools: Tools like dmesg and printk will be essential for debugging any issues that arise during testing.

The Skeleton of a Block Device Module

Let's begin by writing a basic skeleton for our block device kernel module. This will give us a framework upon which we can build our functional block device.

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/blkdev.h>
#include <linux/bio.h>

#define DEVICE_NAME "my_block_device"
#define DEVICE_SIZE (10 * 1024 * 1024) // 10 MB

static struct gendisk *gd;
static struct request_queue *queue;
static void *data;

static void my_request_fn(struct request_queue *q) {
    struct request *req;
    
    while ((req = blk_fetch_request(q)) != NULL) {
        // Here we would generally handle the request
        __blk_end_request_all(req, 0);
    }
}

static int __init my_block_device_init(void) {
    data = vmalloc(DEVICE_SIZE);
    if (!data) {
        return -ENOMEM;
    }

    queue = blk_init_queue(my_request_fn, NULL);
    if (!queue) {
        vfree(data);
        return -ENOMEM;
    }

    gd = alloc_disk(16);
    if (!gd) {
        blk_cleanup_queue(queue);
        vfree(data);
        return -ENOMEM;
    }

    gd->major = register_blkdev(0, DEVICE_NAME);
    gd->first_minor = 0;
    gd->fops = &my_fops;
    gd->queue = queue;
    snprintf(gd->disk_name, 32, DEVICE_NAME);
    set_capacity(gd, DEVICE_SIZE / 512); // Set capacity in 512-byte sectors
    add_disk(gd);
    
    return 0;
}

static void __exit my_block_device_exit(void) {
    del_gendisk(gd);
    put_disk(gd);
    unregister_blkdev(gd->major, DEVICE_NAME);
    blk_cleanup_queue(queue);
    vfree(data);
}

module_init(my_block_device_init);
module_exit(my_block_device_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Simple Block Device");
MODULE_AUTHOR("Your Name");

Explanation of the Code

Includes: We start by including necessary headers. linux/fs.h deals with filesystem-related data structures, while linux/genhd.h and linux/blkdev.h provide structures for block devices.
Global Variables: We define global variables for our gendisk structure gd, the request queue queue, and the data buffer data.
Request Handling: The function my_request_fn is responsible for processing incoming block requests. As a placeholder, we fetch requests from the queue, but you would typically handle reading/writing to your data structure here.
Module Initialization: In my_block_device_init, we allocate memory for our data, initialize the request queue, and allocate and set up our gendisk structure. It also registers the block device with the kernel.
Module Exit: The function my_block_device_exit cleans up the resources allocated during initialization, ensuring we don’t have memory leaks.

Implementing Read and Write Operations

To implement functional read and write operations, we need to enhance the request handling logic in my_request_fn. Each request needs to check the operation type (read/write) and then copy data to or from our data buffer accordingly.

static void my_request_fn(struct request_queue *q) {
    struct request *req;
    
    while ((req = blk_fetch_request(q)) != NULL) {
        struct bio_vec bvec;
        struct bvec_iter iter;
        void *buffer;
        unsigned int len;

        // Process the request
        if (req->cmd_type == REQ_OP_READ) {
            __blk_end_request_all(req, 0);
        } else if (req->cmd_type == REQ_OP_WRITE) {
            __blk_end_request_all(req, 0);
        }

        // Ensure that we finish the request
        bio_for_each_segment(bvec, req->bio, iter) {
            buffer = kmap(bvec.bv_page) + bvec.bv_offset;
            // Perform read/write operations
            len = bvec.bv_len;

            if (req->cmd_type == REQ_OP_READ) {
                memcpy(buffer, data + (req->start * 512), len);
            } else if (req->cmd_type == REQ_OP_WRITE) {
                memcpy(data + (req->start * 512), buffer, len);
            }

            kunmap(bvec.bv_page);
            __blk_end_request_all(req, len);
        }
    }
}

Finalizing Your Block Device

After implementing the read/write functionality, verify and expand your module by adding error-checking mechanisms, ensuring the integrity of copied data, and managing concurrency issues, especially if your module is accessed from multiple processors.

Also, remember to handle the allocation of space carefully. The example above uses vmalloc, which is simple but not the most efficient method under certain circumstances. In production modules, you may wish to consider block allocation strategies that help maintain performance and avoid fragmentation.

Testing Your Block Device

Once your module is complete, compiling it is straightforward. Use make to build your module and insmod to insert it into the kernel. After insertion, check for its availability using lsblk or fdisk -l. Testing your block device’s read and write capabilities can be done using a simple filesystem like mkfs.ext4 followed by mount.

Conclusion

Creating a block device in the Linux kernel is an invaluable skill for developers working on system-level programming. This article has laid the groundwork laid for you to understand and implement a basic block device. As you experiment more and start adding features, try to implement features like caching, advanced error handling, or even integrating it with the Linux I/O scheduler.

Keep learning and experimenting, and soon you'll be creating robust kernel modules that extend the capabilities of the Linux operating system!

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