Commit 770fe30a authored by Kay Sievers's avatar Kay Sievers Committed by Jens Axboe
Browse files

loop: add management interface for on-demand device allocation 



Loop devices today have a fixed pre-allocated number of usually 8.
The number can only be changed at module init time. To find a free
device to use, /dev/loop%i needs to be scanned, and all devices need
to be opened until a free one is possibly found.

This adds a new /dev/loop-control device node, that allows to
dynamically find or allocate a free device, and to add and remove loop
devices from the running system:
 LOOP_CTL_ADD adds a specific device. Arg is the number
 of the device. It returns the device i or a negative
 error code.

 LOOP_CTL_REMOVE removes a specific device, Arg is the
 number the device. It returns the device i or a negative
 error code.

 LOOP_CTL_GET_FREE finds the next unbound device or allocates
 a new one. No arg is given. It returns the device i or a
 negative error code.

The loop kernel module gets automatically loaded when
/dev/loop-control is accessed the first time. The alias
specified in the module, instructs udev to create this
'dead' device node, even when the module is not loaded.

Example:
 cfd = open("/dev/loop-control", O_RDWR);

 # add a new specific loop device
 err = ioctl(cfd, LOOP_CTL_ADD, devnr);

 # remove a specific loop device
 err = ioctl(cfd, LOOP_CTL_REMOVE, devnr);

 # find or allocate a free loop device to use
 devnr = ioctl(cfd, LOOP_CTL_GET_FREE);

 sprintf(loopname, "/dev/loop%i", devnr);
 ffd = open("backing-file", O_RDWR);
 lfd = open(loopname, O_RDWR);
 err = ioctl(lfd, LOOP_SET_FD, ffd);

Cc: Tejun Heo <tj@kernel.org>
Cc: Karel Zak  <kzak@redhat.com>
Signed-off-by: default avatarKay Sievers <kay.sievers@vrfy.org>
Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
parent 34dd82af

drivers/block/loop.c

+116 −4
Original line number Diff line number Diff line
@@ -75,7 +75,7 @@ 
#include <linux/kthread.h>

#include <linux/splice.h>

#include <linux/sysfs.h>



#include <linux/miscdevice.h>

#include <asm/uaccess.h>



static DEFINE_IDR(loop_index_idr);
@@ -1478,13 +1478,22 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode, 


static int lo_open(struct block_device *bdev, fmode_t mode)

{

	struct loop_device *lo = bdev->bd_disk->private_data;

	struct loop_device *lo;

	int err = 0;



	mutex_lock(&loop_index_mutex);

	lo = bdev->bd_disk->private_data;

	if (!lo) {

		err = -ENXIO;

		goto out;

	}



	mutex_lock(&lo->lo_ctl_mutex);

	lo->lo_refcnt++;

	mutex_unlock(&lo->lo_ctl_mutex);



	return 0;

out:

	mutex_unlock(&loop_index_mutex);

	return err;

}



static int lo_release(struct gendisk *disk, fmode_t mode)
@@ -1603,6 +1612,13 @@ static int loop_add(struct loop_device **l, int i) 
			idr_remove(&loop_index_idr, m);

			err = -EEXIST;

		}

	} else if (i == -1) {

		int m;



		/* get next free nr */

		err = idr_get_new(&loop_index_idr, lo, &m);

		if (err >= 0)

			i = m;

	} else {

		err = -EINVAL;

	}
@@ -1648,16 +1664,41 @@ static void loop_remove(struct loop_device *lo) 
	kfree(lo);

}



static int find_free_cb(int id, void *ptr, void *data)

{

	struct loop_device *lo = ptr;

	struct loop_device **l = data;



	if (lo->lo_state == Lo_unbound) {

		*l = lo;

		return 1;

	}

	return 0;

}



static int loop_lookup(struct loop_device **l, int i)

{

	struct loop_device *lo;

	int ret = -ENODEV;



	if (i < 0) {

		int err;



		err = idr_for_each(&loop_index_idr, &find_free_cb, &lo);

		if (err == 1) {

			*l = lo;

			ret = lo->lo_number;

		}

		goto out;

	}



	/* lookup and return a specific i */

	lo = idr_find(&loop_index_idr, i);

	if (lo) {

		*l = lo;

		ret = lo->lo_number;

	}

out:

	return ret;

}
@@ -1681,11 +1722,76 @@ static struct kobject *loop_probe(dev_t dev, int *part, void *data) 
	return kobj;

}



static long loop_control_ioctl(struct file *file, unsigned int cmd,

			       unsigned long parm)

{

	struct loop_device *lo;

	int ret = -ENOSYS;



	mutex_lock(&loop_index_mutex);

	switch (cmd) {

	case LOOP_CTL_ADD:

		ret = loop_lookup(&lo, parm);

		if (ret >= 0) {

			ret = -EEXIST;

			break;

		}

		ret = loop_add(&lo, parm);

		break;

	case LOOP_CTL_REMOVE:

		ret = loop_lookup(&lo, parm);

		if (ret < 0)

			break;

		mutex_lock(&lo->lo_ctl_mutex);

		if (lo->lo_state != Lo_unbound) {

			ret = -EBUSY;

			mutex_unlock(&lo->lo_ctl_mutex);

			break;

		}

		if (lo->lo_refcnt > 0) {

			ret = -EBUSY;

			mutex_unlock(&lo->lo_ctl_mutex);

			break;

		}

		lo->lo_disk->private_data = NULL;

		mutex_unlock(&lo->lo_ctl_mutex);

		idr_remove(&loop_index_idr, lo->lo_number);

		loop_remove(lo);

		break;

	case LOOP_CTL_GET_FREE:

		ret = loop_lookup(&lo, -1);

		if (ret >= 0)

			break;

		ret = loop_add(&lo, -1);

	}

	mutex_unlock(&loop_index_mutex);



	return ret;

}



static const struct file_operations loop_ctl_fops = {

	.open		= nonseekable_open,

	.unlocked_ioctl	= loop_control_ioctl,

	.compat_ioctl	= loop_control_ioctl,

	.owner		= THIS_MODULE,

	.llseek		= noop_llseek,

};



static struct miscdevice loop_misc = {

	.minor		= LOOP_CTRL_MINOR,

	.name		= "loop-control",

	.fops		= &loop_ctl_fops,

};



MODULE_ALIAS_MISCDEV(LOOP_CTRL_MINOR);

MODULE_ALIAS("devname:loop-control");



static int __init loop_init(void)

{

	int i, nr;

	unsigned long range;

	struct loop_device *lo;

	int err;



	/*

	 * loop module now has a feature to instantiate underlying device
@@ -1702,6 +1808,10 @@ static int __init loop_init(void) 
	 *     device on-demand.

	 */



	err = misc_register(&loop_misc);

	if (err < 0)

		return err;



	part_shift = 0;

	if (max_part > 0) {

		part_shift = fls(max_part);
@@ -1767,6 +1877,8 @@ static void __exit loop_exit(void) 


	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), range);

	unregister_blkdev(LOOP_MAJOR, "loop");



	misc_deregister(&loop_misc);

}



module_init(loop_init);

include/linux/loop.h

+4 −0
Original line number Diff line number Diff line
@@ -160,4 +160,8 @@ int loop_unregister_transfer(int number); 
#define LOOP_CHANGE_FD		0x4C06

#define LOOP_SET_CAPACITY	0x4C07



/* /dev/loop-control interface */

#define LOOP_CTL_ADD		0x4C80

#define LOOP_CTL_REMOVE		0x4C81

#define LOOP_CTL_GET_FREE	0x4C82

#endif

include/linux/miscdevice.h

+1 −0
Original line number Diff line number Diff line
@@ -40,6 +40,7 @@ 
#define BTRFS_MINOR		234

#define AUTOFS_MINOR		235

#define MAPPER_CTRL_MINOR	236

#define LOOP_CTRL_MINOR		237

#define MISC_DYNAMIC_MINOR	255



struct device;

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