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    Subject[PATCH][v2] Modify loop device to be able to manage partitions of the disk image
    Date
    This patch allows to use loop device with partitionned disk image.

    Original behavior of loop is not modified.

    A new parameter is introduced to define how many partition we want to be
    able to manage per loop device. This parameter is "loop_max_part".

    For instance, to manage 63 partitions / loop device, we will do:
    # modprobe loop loop_max_part=63
    # ls -l /dev/loop?
    brw-rw---- 1 root disk 7, 0 2008-03-05 14:55 /dev/loop0
    brw-rw---- 1 root disk 7, 64 2008-03-05 14:55 /dev/loop1
    brw-rw---- 1 root disk 7, 128 2008-03-05 14:55 /dev/loop2
    brw-rw---- 1 root disk 7, 192 2008-03-05 14:55 /dev/loop3
    brw-rw---- 1 root disk 7, 256 2008-03-05 14:55 /dev/loop4
    brw-rw---- 1 root disk 7, 320 2008-03-05 14:55 /dev/loop5
    brw-rw---- 1 root disk 7, 384 2008-03-05 14:55 /dev/loop6
    brw-rw---- 1 root disk 7, 448 2008-03-05 14:55 /dev/loop7

    And to attach a raw partitionned disk image, the original losetup is used:

    # losetup -f etch.img
    EXT3 FS on loop0p1, internal journal
    EXT3-fs: mounted filesystem with ordered data mode.
    loop: module loaded
    loop0: p1 p2 < p5 >
    # ls -l /dev/loop?*
    brw-rw---- 1 root disk 7, 0 2008-03-05 14:55 /dev/loop0
    brw-rw---- 1 root disk 7, 1 2008-03-05 14:57 /dev/loop0p1
    brw-rw---- 1 root disk 7, 2 2008-03-05 14:57 /dev/loop0p2
    brw-rw---- 1 root disk 7, 5 2008-03-05 14:57 /dev/loop0p5
    brw-rw---- 1 root disk 7, 64 2008-03-05 14:55 /dev/loop1
    brw-rw---- 1 root disk 7, 128 2008-03-05 14:55 /dev/loop2
    brw-rw---- 1 root disk 7, 192 2008-03-05 14:55 /dev/loop3
    brw-rw---- 1 root disk 7, 256 2008-03-05 14:55 /dev/loop4
    brw-rw---- 1 root disk 7, 320 2008-03-05 14:55 /dev/loop5
    brw-rw---- 1 root disk 7, 384 2008-03-05 14:55 /dev/loop6
    brw-rw---- 1 root disk 7, 448 2008-03-05 14:55 /dev/loop7
    # mount /dev/loop0p1 /mnt
    kjournald starting. Commit interval 5 seconds
    EXT3 FS on loop0p1, internal journal
    EXT3-fs: mounted filesystem with ordered data mode.
    # ls /mnt
    bench cdrom home lib mnt root srv usr
    bin dev initrd lost+found opt sbin sys var
    boot etc initrd.img media proc selinux tmp vmlinuz
    # umount /mnt
    # losetup -d /dev/loop0

    Of course, the same behavior can be done using kpartx on a loop device,
    but modifying loop avoids to stack several layers of block device (loop +
    device mapper), this is a very light modification (40% of modifications
    are to manage the new parameter).

    Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
    ---
    Documentation/kernel-parameters.txt | 4 +++
    drivers/block/loop.c | 39 ++++++++++++++++++++++++++++++----
    2 files changed, 38 insertions(+), 5 deletions(-)

    diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
    index 9a5b665..7f2fd52 100644
    --- a/Documentation/kernel-parameters.txt
    +++ b/Documentation/kernel-parameters.txt
    @@ -1075,6 +1075,10 @@ and is between 256 and 4096 characters. It is defined in the file
    be mounted
    Format: <1-256>

    + loop_max_part= [LOOP] Maximum number of partitions per loopback device.
    + Should be greater than 0, the maximum value depends
    + on max_loop.
    +
    maxcpus= [SMP] Maximum number of processors that an SMP kernel
    should make use of. maxcpus=n : n >= 0 limits the
    kernel to using 'n' processors. n=0 is a special case,
    diff --git a/drivers/block/loop.c b/drivers/block/loop.c
    index 91ebb00..84fa98f 100644
    --- a/drivers/block/loop.c
    +++ b/drivers/block/loop.c
    @@ -82,6 +82,9 @@
    static LIST_HEAD(loop_devices);
    static DEFINE_MUTEX(loop_devices_mutex);

    +static int loop_max_part;
    +static int part_shift;
    +
    /*
    * Transfer functions
    */
    @@ -692,6 +695,8 @@ static int loop_change_fd(struct loop_device *lo, struct file *lo_file,
    goto out_putf;

    fput(old_file);
    + if (loop_max_part > 0)
    + ioctl_by_bdev(bdev, BLKRRPART, 0);
    return 0;

    out_putf:
    @@ -819,6 +824,8 @@ static int loop_set_fd(struct loop_device *lo, struct file *lo_file,
    }
    lo->lo_state = Lo_bound;
    wake_up_process(lo->lo_thread);
    + if (loop_max_part > 0)
    + ioctl_by_bdev(bdev, BLKRRPART, 0);
    return 0;

    out_clr:
    @@ -1360,6 +1367,8 @@ static struct block_device_operations lo_fops = {
    static int max_loop;
    module_param(max_loop, int, 0);
    MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
    +module_param(loop_max_part, int, 0);
    +MODULE_PARM_DESC(loop_max_part, "Maximum number of partitions per loop device");
    MODULE_LICENSE("GPL");
    MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);

    @@ -1412,7 +1421,7 @@ static struct loop_device *loop_alloc(int i)
    if (!lo->lo_queue)
    goto out_free_dev;

    - disk = lo->lo_disk = alloc_disk(1);
    + disk = lo->lo_disk = alloc_disk(1 << part_shift);
    if (!disk)
    goto out_free_queue;

    @@ -1422,7 +1431,7 @@ static struct loop_device *loop_alloc(int i)
    init_waitqueue_head(&lo->lo_event);
    spin_lock_init(&lo->lo_lock);
    disk->major = LOOP_MAJOR;
    - disk->first_minor = i;
    + disk->first_minor = i << part_shift;
    disk->fops = &lo_fops;
    disk->private_data = lo;
    disk->queue = lo->lo_queue;
    @@ -1502,7 +1511,12 @@ static int __init loop_init(void)
    * themselves and have kernel automatically instantiate actual
    * device on-demand.
    */
    - if (max_loop > 1UL << MINORBITS)
    +
    + part_shift = 0;
    + if (loop_max_part > 0)
    + part_shift = fls(loop_max_part);
    +
    + if (max_loop > 1UL << (MINORBITS - part_shift))
    return -EINVAL;

    if (max_loop) {
    @@ -1510,7 +1524,7 @@ static int __init loop_init(void)
    range = max_loop;
    } else {
    nr = 8;
    - range = 1UL << MINORBITS;
    + range = 1UL << (MINORBITS - part_shift);
    }

    if (register_blkdev(LOOP_MAJOR, "loop"))
    @@ -1549,7 +1563,7 @@ static void __exit loop_exit(void)
    unsigned long range;
    struct loop_device *lo, *next;

    - range = max_loop ? max_loop : 1UL << MINORBITS;
    + range = max_loop ? max_loop : 1UL << (MINORBITS - part_shift);

    list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
    loop_del_one(lo);
    @@ -1569,4 +1583,19 @@ static int __init max_loop_setup(char *str)
    }

    __setup("max_loop=", max_loop_setup);
    +
    +static int __init max_part_setup(char *str)
    +{
    + loop_max_part = simple_strtol(str, NULL, 0);
    + if (loop_max_part > (1UL << (MINORBITS - 1))) {
    + /* we must keep at least one bit for loop device number */
    + printk(KERN_ERR
    + "loop: loop_max_part cannot be greater than %d\n",
    + 1UL << (MINORBITS - 1));
    + return 0;
    + }
    + return 1;
    +}
    +
    +__setup("loop_max_part=", max_part_setup);
    #endif
    --
    1.5.2.4


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