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Date: Wed, 19 Mar 2008 21:32:32 +0100
From: Laurent Vivier <Laurent.Vivier@...l.net>
To: Bill Davidsen <davidsen@....com>
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
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@...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
>
>
--
------- Laurent.Vivier@...l.net -------
"The best way to predict the future
is to invent it." - Alan Kay
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