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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: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
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1 parent 34dd82a commit 770fe30a46a12b6fb6b63fbe1737654d28e84844 @kaysievers kaysievers committed with Jens Axboe Jul 31, 2011
Showing with 121 additions and 4 deletions.
  1. +116 −4 drivers/block/loop.c
  2. +4 −0 include/linux/loop.h
  3. +1 −0 include/linux/miscdevice.h
View
@@ -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);
View
@@ -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
@@ -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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