Re: [PATCH][v4] Modify loop device to be able to manage partitions of the disk image

[Randy Dunlap <randy.dunlap@oracle.com>]

On Wed, 19 Mar 2008 13:36:07 +0100 Laurent Vivier wrote:

> 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 "max_part".

What happened to the update to Documentation/kernel-parameters.txt
that was in v3?


> For instance, to manage 63 partitions / loop device, we will do:
> # modprobe 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
> # 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
> # 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>
> ---
>  drivers/block/loop.c |   26 +++++++++++++++++++++-----
>  1 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 91ebb00..f7f1635 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 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 (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 (max_part > 0)
> +		ioctl_by_bdev(bdev, BLKRRPART, 0);
>  	return 0;
>  
>  out_clr:
> @@ -919,6 +926,8 @@ static int loop_clr_fd(struct loop_device *lo, struct block_device *bdev)
>  	fput(filp);
>  	/* This is safe: open() is still holding a reference. */
>  	module_put(THIS_MODULE);
> +	if (max_part > 0)
> +		ioctl_by_bdev(bdev, BLKRRPART, 0);
>  	return 0;
>  }
>  
> @@ -1360,6 +1369,8 @@ 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(max_part, int, 0);
> +MODULE_PARM_DESC(max_part, "Maximum number of partitions per loop device");
>  MODULE_LICENSE("GPL");
>  MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
>  
> @@ -1412,7 +1423,7 @@ 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 +1433,7 @@ 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 +1513,12 @@ 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 (max_part > 0)
> +		part_shift = fls(max_part);
> +
> +	if (max_loop > 1UL << (MINORBITS - part_shift))
>  		return -EINVAL;
>  
>  	if (max_loop) {
> @@ -1510,7 +1526,7 @@ 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 +1565,7 @@ 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);
> -- 


---
~Randy