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>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f58532e..5c9edf9 100644 (file)
--- 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);

diff --git a/include/linux/loop.h b/include/linux/loop.h
index 5f08d18..683d698 100644 (file)
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -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

diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index 18fd130..c309b1e 100644 (file)
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -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;