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    Subject[PATCH][RESEND][v3] Modify loop device to be able to manage partitions of the disk image
    Date
    v3 is an updated version of v2, replacing a "%d" by a "%lu".

    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..16b2aa6 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 %lu\n",
    + 1UL << (MINORBITS - 1));
    + return 0;
    + }
    + return 1;
    +}
    +
    +__setup("loop_max_part=", max_part_setup);
    #endif
    --
    1.5.2.4


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