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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parent
34dd82afd2
commit
770fe30a46
3 changed files with 121 additions and 4 deletions
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@ -75,7 +75,7 @@
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#include <linux/kthread.h>
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#include <linux/splice.h>
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#include <linux/sysfs.h>
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#include <linux/miscdevice.h>
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#include <asm/uaccess.h>
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static DEFINE_IDR(loop_index_idr);
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@ -1478,13 +1478,22 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
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static int lo_open(struct block_device *bdev, fmode_t mode)
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{
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struct loop_device *lo = bdev->bd_disk->private_data;
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struct loop_device *lo;
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int err = 0;
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mutex_lock(&loop_index_mutex);
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lo = bdev->bd_disk->private_data;
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if (!lo) {
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err = -ENXIO;
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goto out;
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}
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mutex_lock(&lo->lo_ctl_mutex);
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lo->lo_refcnt++;
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mutex_unlock(&lo->lo_ctl_mutex);
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return 0;
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out:
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mutex_unlock(&loop_index_mutex);
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return err;
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}
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static int lo_release(struct gendisk *disk, fmode_t mode)
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@ -1603,6 +1612,13 @@ static int loop_add(struct loop_device **l, int i)
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idr_remove(&loop_index_idr, m);
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err = -EEXIST;
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}
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} else if (i == -1) {
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int m;
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/* get next free nr */
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err = idr_get_new(&loop_index_idr, lo, &m);
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if (err >= 0)
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i = m;
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} else {
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err = -EINVAL;
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}
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@ -1648,16 +1664,41 @@ static void loop_remove(struct loop_device *lo)
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kfree(lo);
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}
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static int find_free_cb(int id, void *ptr, void *data)
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{
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struct loop_device *lo = ptr;
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struct loop_device **l = data;
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if (lo->lo_state == Lo_unbound) {
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*l = lo;
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return 1;
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}
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return 0;
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}
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static int loop_lookup(struct loop_device **l, int i)
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{
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struct loop_device *lo;
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int ret = -ENODEV;
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if (i < 0) {
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int err;
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err = idr_for_each(&loop_index_idr, &find_free_cb, &lo);
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if (err == 1) {
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*l = lo;
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ret = lo->lo_number;
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}
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goto out;
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}
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/* lookup and return a specific i */
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lo = idr_find(&loop_index_idr, i);
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if (lo) {
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*l = lo;
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ret = lo->lo_number;
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}
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out:
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return ret;
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}
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@ -1681,11 +1722,76 @@ static struct kobject *loop_probe(dev_t dev, int *part, void *data)
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return kobj;
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}
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static long loop_control_ioctl(struct file *file, unsigned int cmd,
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unsigned long parm)
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{
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struct loop_device *lo;
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int ret = -ENOSYS;
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mutex_lock(&loop_index_mutex);
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switch (cmd) {
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case LOOP_CTL_ADD:
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ret = loop_lookup(&lo, parm);
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if (ret >= 0) {
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ret = -EEXIST;
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break;
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}
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ret = loop_add(&lo, parm);
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break;
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case LOOP_CTL_REMOVE:
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ret = loop_lookup(&lo, parm);
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if (ret < 0)
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break;
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mutex_lock(&lo->lo_ctl_mutex);
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if (lo->lo_state != Lo_unbound) {
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ret = -EBUSY;
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mutex_unlock(&lo->lo_ctl_mutex);
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break;
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}
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if (lo->lo_refcnt > 0) {
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ret = -EBUSY;
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mutex_unlock(&lo->lo_ctl_mutex);
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break;
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}
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lo->lo_disk->private_data = NULL;
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mutex_unlock(&lo->lo_ctl_mutex);
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idr_remove(&loop_index_idr, lo->lo_number);
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loop_remove(lo);
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break;
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case LOOP_CTL_GET_FREE:
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ret = loop_lookup(&lo, -1);
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if (ret >= 0)
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break;
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ret = loop_add(&lo, -1);
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}
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mutex_unlock(&loop_index_mutex);
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return ret;
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}
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static const struct file_operations loop_ctl_fops = {
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.open = nonseekable_open,
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.unlocked_ioctl = loop_control_ioctl,
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.compat_ioctl = loop_control_ioctl,
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.owner = THIS_MODULE,
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.llseek = noop_llseek,
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};
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static struct miscdevice loop_misc = {
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.minor = LOOP_CTRL_MINOR,
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.name = "loop-control",
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.fops = &loop_ctl_fops,
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};
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MODULE_ALIAS_MISCDEV(LOOP_CTRL_MINOR);
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MODULE_ALIAS("devname:loop-control");
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static int __init loop_init(void)
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{
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int i, nr;
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unsigned long range;
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struct loop_device *lo;
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int err;
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/*
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* loop module now has a feature to instantiate underlying device
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@ -1702,6 +1808,10 @@ static int __init loop_init(void)
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* device on-demand.
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*/
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err = misc_register(&loop_misc);
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if (err < 0)
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return err;
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part_shift = 0;
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if (max_part > 0) {
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part_shift = fls(max_part);
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@ -1767,6 +1877,8 @@ static void __exit loop_exit(void)
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blk_unregister_region(MKDEV(LOOP_MAJOR, 0), range);
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unregister_blkdev(LOOP_MAJOR, "loop");
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misc_deregister(&loop_misc);
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}
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module_init(loop_init);
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@ -160,4 +160,8 @@ int loop_unregister_transfer(int number);
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#define LOOP_CHANGE_FD 0x4C06
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#define LOOP_SET_CAPACITY 0x4C07
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/* /dev/loop-control interface */
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#define LOOP_CTL_ADD 0x4C80
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#define LOOP_CTL_REMOVE 0x4C81
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#define LOOP_CTL_GET_FREE 0x4C82
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#endif
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@ -40,6 +40,7 @@
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#define BTRFS_MINOR 234
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#define AUTOFS_MINOR 235
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#define MAPPER_CTRL_MINOR 236
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#define LOOP_CTRL_MINOR 237
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#define MISC_DYNAMIC_MINOR 255
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struct device;
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