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>

1 files changed, 116 insertions, 4 deletions

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f58532e7777..5c9edf94487 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -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);