Re: [PATCH][RESEND][v3] Modify loop device to be able to manage partitions of the disk image

[Laurent Vivier <Laurent.Vivier@bull.net>]

Le mercredi 19 mars 2008 à 16:24 -0400, Bill Davidsen a écrit :
> Laurent Vivier wrote:
> > v3 is an updated version of v2, replacing a "%d" by a "%lu".
> > 
> > This patch allows to use loop device with partitionned disk image.
> > 
> How does this compare in overhead to using nbd?

What do you mean ?

NBD doesn't manage partitions... but I also have a patch to do that.
NBD implies an NBD server and an NBD client in userspace, so loop is
better when disk image is in a raw format.

Laurent

> > 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
> 
> 
-- 
------- Laurent.Vivier@bull.net  -------
  "The best way to predict the future 
      is to invent it." - Alan Kay