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Date: Wed, 19 Mar 2008 16:24:19 -0400
From: Bill Davidsen <davidsen@....com>
To: Laurent Vivier <Laurent.Vivier@...l.net>
CC: axboe@...e.de, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.org
Subject: Re: [PATCH][RESEND][v3] Modify loop device to be able to manage partitions
of the disk image
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?
> 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@...l.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
--
Bill Davidsen <davidsen@....com>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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