[Qemu-devel] [RFC][PATCH] Modify loop device to be able to manage partitions of the image disk

[Laurent Vivier]

As it should be useful to be able to mount partition from a disk image, (and as 
I need a break in my bug hunting) I've modified the loop driver to mount raw 
disk image.

To not break original loop device, as we have to change minor numbers to manage 
partitions, a new parameter is added to the module:

max_part which define the maximum number of partitions by loop device.

By default max_part is equal to 1.

The patch has been created against the current KVM kernel source tree 
(2.6.24-rc6).

Example:

# insmod drivers/block/loop.ko
# ls -ld /dev/loop*
drwxr-xr-x 2 root root     60 2008-01-15 18:49 /dev/loop
brw-rw---- 1 root disk 7, 0 2008-01-15 19:07 /dev/loop0
brw-rw---- 1 root disk 7, 1 2008-01-15 19:07 /dev/loop1
brw-rw---- 1 root disk 7, 2 2008-01-15 19:07 /dev/loop2
brw-rw---- 1 root disk 7, 3 2008-01-15 19:07 /dev/loop3
brw-rw---- 1 root disk 7, 4 2008-01-15 19:07 /dev/loop4
brw-rw---- 1 root disk 7, 5 2008-01-15 19:07 /dev/loop5
brw-rw---- 1 root disk 7, 6 2008-01-15 19:07 /dev/loop6
brw-rw---- 1 root disk 7, 7 2008-01-15 19:07 /dev/loop7
# rmmod loop
# insmod drivers/block/loop.ko max_part=16
# ls -ld /dev/loop*
drwxr-xr-x 2 root root     60 2008-01-15 18:49 /dev/loop
brw-rw---- 1 root disk 7,   0 2008-01-15 19:08 /dev/loop0
brw-rw---- 1 root disk 7,  16 2008-01-15 19:08 /dev/loop1
brw-rw---- 1 root disk 7,  32 2008-01-15 19:08 /dev/loop2
brw-rw---- 1 root disk 7,  48 2008-01-15 19:08 /dev/loop3
brw-rw---- 1 root disk 7,  64 2008-01-15 19:08 /dev/loop4
brw-rw---- 1 root disk 7,  80 2008-01-15 19:08 /dev/loop5
brw-rw---- 1 root disk 7,  96 2008-01-15 19:08 /dev/loop6
brw-rw---- 1 root disk 7, 112 2008-01-15 19:08 /dev/loop7
# losetup /dev/loop0 etch.img
# ls -ld /dev/loop*
drwxr-xr-x 2 root root     60 2008-01-15 18:49 /dev/loop
brw-rw---- 1 root disk 7,   0 2008-01-15 19:08 /dev/loop0
brw-rw---- 1 root disk 7,   1 2008-01-15 19:09 /dev/loop0p1
brw-rw---- 1 root disk 7,   2 2008-01-15 19:09 /dev/loop0p2
brw-rw---- 1 root disk 7,   5 2008-01-15 19:09 /dev/loop0p5
brw-rw---- 1 root disk 7,  16 2008-01-15 19:08 /dev/loop1
brw-rw---- 1 root disk 7,  32 2008-01-15 19:08 /dev/loop2
brw-rw---- 1 root disk 7,  48 2008-01-15 19:08 /dev/loop3
brw-rw---- 1 root disk 7,  64 2008-01-15 19:08 /dev/loop4
brw-rw---- 1 root disk 7,  80 2008-01-15 19:08 /dev/loop5
brw-rw---- 1 root disk 7,  96 2008-01-15 19:08 /dev/loop6
brw-rw---- 1 root disk 7, 112 2008-01-15 19:08 /dev/loop7
# mount /dev/loop0p1 /mnt
# ls /m  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
# rmmod loop

All comments are welcome, perhaps it is stupid idea...

Signed-off-by: Laurent Vivier <address@hidden>
---
 drivers/block/loop.c |   43 ++++++++++++++++++++++++++++++++++++-------
 1 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 56e2304..f601633 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -82,6 +82,8 @@
 static LIST_HEAD(loop_devices);
 static DEFINE_MUTEX(loop_devices_mutex);
 
+static int part_shift;
+
 /*
  * Transfer functions
  */
@@ -819,6 +821,7 @@ static int loop_set_fd(struct loop_device *lo, struct file 
*lo_file,
        }
        lo->lo_state = Lo_bound;
        wake_up_process(lo->lo_thread);
+       ioctl_by_bdev(bdev, BLKRRPART, 0);
        return 0;
 
 out_clr:
@@ -1352,6 +1355,9 @@ 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");
+static int max_part = 1;
+module_param(max_part, int, 0);
+MODULE_PARM_DESC(max_part, "Maximum number of partition by loop device");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
 
@@ -1404,7 +1410,7 @@ 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;
 
@@ -1414,7 +1420,7 @@ 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;
@@ -1466,7 +1472,7 @@ static struct kobject *loop_probe(dev_t dev, int *part, 
void *data)
        struct kobject *kobj;
 
        mutex_lock(&loop_devices_mutex);
-       lo = loop_init_one(dev & MINORMASK);
+       lo = loop_init_one((dev & MINORMASK) & 0x0F);
        kobj = lo ? get_disk(lo->lo_disk) : ERR_PTR(-ENOMEM);
        mutex_unlock(&loop_devices_mutex);
 
@@ -1477,7 +1483,7 @@ static struct kobject *loop_probe(dev_t dev, int *part, 
void *data)
 static int __init loop_init(void)
 {
        int i, nr;
-       unsigned long range;
+       unsigned long range, bitfield;
        struct loop_device *lo, *next;
 
        /*
@@ -1494,7 +1500,11 @@ static int __init loop_init(void)
         *     themselves and have kernel automatically instantiate actual
         *     device on-demand.
         */
-       if (max_loop > 1UL << MINORBITS)
+
+       bitfield = max_part;
+       part_shift = find_first_bit(&bitfield, sizeof(bitfield));
+
+       if (max_loop > 1UL << (MINORBITS - part_shift))
                return -EINVAL;
 
        if (max_loop) {
@@ -1502,7 +1512,7 @@ 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"))
@@ -1541,7 +1551,7 @@ 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);
@@ -1561,4 +1571,23 @@ static int __init max_loop_setup(char *str)
 }
 
 __setup("max_loop=", max_loop_setup);
+
+static int __init max_part_setup(char *str)
+{
+       max_part = simple_strtol(str, NULL, 0);
+       if (max_part < 1) {
+               /* there is at least one partition */ 
+               printk(KERN_ERR "loop: max_part cannot be lesser than 1\n");
+               return 0;
+       }
+        if (max_part > MINORBITS - 1) {
+               /* we must keep at least one bit for loop device number */
+               printk(KERN_ERR "loop: max_part cannot be greater than %d\n",
+                       MINORBITS - 1);
+               return 0;
+       }
+       return 1;
+}
+
+__setup("max_part=", max_part_setup);
 #endif
-- 
1.5.2.4