How to read a sector using a bio request in Linux kernel

Question

How do I create my own bio request to read a sector from the disk drive ?

I am trying the following but it freezes the system.

static void read_bio()
{
    struct bio *b;
    struct page *p;

    b = bio_alloc(GFP_KERNEL, 1);
    if (!b) {
        printk(KERN_INFO "bio allocation failed\n");
    }
    bio_init(b);

    b->bi_sector = 10000;
    b->bi_bdev = bd;    /* "/dev/sda1" */
    b->bi_end_io = bio_end_clone;

    p = alloc_page(GFP_KERNEL);
    if (!p) {
        printk(KERN_INFO "page allocation failed\n");
    }
    bio_add_page(b, p, PAGE_SIZE, 0);
    b->bi_private = p;

    bio_get(b);
    submit_bio(READ, b);
    printk(KERN_DEBUG "submit read request\n");
}

Answer 1

It is an old question, but anyway here is the code for reading, I hope it will help someone:

int readPage(struct block_device *device, sector_t sector, int size,
     struct page *page)
{
    int ret;
    struct completion event;
    struct bio *bio = bio_alloc(device, 1, REQ_OP_READ, GFP_NOIO);

    bio->bi_iter.bi_sector = sector;

    bio_add_page(bio, page, size, 0);
    init_completion(&event);

    bio->bi_private = &event;
    bio->bi_end_io = readComplete;

    submit_bio(bio);
    wait_for_completion(&event);
    ret = test_bit(BIO_UPTODATE, &bio->bi_flags);

    bio_put(bio);
    return ret;
}

And for writing:

void writePage(struct block_device *device,
    sector_t sector, int size, struct page *page)
{
    struct bio *bio = bio_alloc(device, 1, REQ_OP_WRITE, GFP_NOIO);
    bio->bi_iter.bi_sector = sector;

    bio_add_page(bio, page, size, 0);
    bio->bi_end_io = writeComplete;

    submit_bio(bio);
}

page can be allocated with alloc_page(GFP_KERNEL). Also for changing data in page use page_address(page). It returns void* so you can interpret that pointer as whatever you want.