[PATCH] loop: add management interface for on-demand deviceallocation

From: Kay Sievers
Date: Tue Jul 26 2011 - 07:50:43 EST

From: Kay Sievers <kay.sievers@xxxxxxxx>
Subject: 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 init time. To find a free device
to use, /dev/loop%i needs to be scanned and all devices need to be
opened.

This adds a new /dev/loop-control device node allows to dynamically
request new, 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 nr or a negative
error code.

LOOP_CTL_REMOVE removes a specific device, Arg is the
number the device. It returns the device nr 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 nr 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
device node, even when the module is not loaded.

A kernel config option BLK_DEV_LOOP_MIN_COUNT is introduced
to allow setups without any pre-created dead loop device,
if none is needed. The default is the historic value of 8.

The linked list to keep track of allocated loop devices is
replaced by a more efficient idr index.

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@xxxxxxxxxx>
Cc: Karel Zak <kzak@xxxxxxxxxx>
Signed-off-by: Kay Sievers <kay.sievers@xxxxxxxx>
---

Documentation/kernel-parameters.txt | 13 -
drivers/block/Kconfig | 15 +
drivers/block/loop.c | 361 +++++++++++++++++++++++-------------
include/linux/loop.h | 10
include/linux/miscdevice.h | 1
5 files changed, 266 insertions(+), 134 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 40cc653..504471b 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1289,6 +1289,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
kernel log messages and is useful when debugging
kernel boot problems.

+ loop.max_loop= [LOOP] The number of loop block devices that get
+ (max_loop) unconditionally pre-created at init time. The default
+ number is configured by BLK_DEV_LOOP_MIN_COUNT. Loop
+ devices can be requested on-demand with the
+ /dev/loop-control interface.
+
+ loop.max_part= [LOOP] Maximum possible number of partitions to create
+ per loop device.
+
lp=0 [LP] Specify parallel ports to use, e.g,
lp=port[,port...] lp=none,parport0 (lp0 not configured, lp1 uses
lp=reset first parallel port). 'lp=0' disables the
@@ -1340,10 +1349,6 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
it is equivalent to "nosmp", which also disables
the IO APIC.

- max_loop= [LOOP] Maximum number of loopback devices that can
- be mounted
- Format: <1-256>
-
mcatest= [IA-64]

mce [X86-32] Machine Check Exception
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 717d6e4..a382ab5 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -254,7 +254,20 @@ config BLK_DEV_LOOP
To compile this driver as a module, choose M here: the
module will be called loop.

- Most users will answer N here.
+config BLK_DEV_LOOP_MIN_COUNT
+ int "Number of loop devices to pre-create at init time"
+ depends on BLK_DEV_LOOP
+ default 8
+ help
+ Number of loop devices to be unconditionally pre-created at
+ init time.
+
+ This default value can be overwritten on the kernel command
+ line or with module-parameter loop.max_loop.
+
+ The historic default is 8, if a late 2011 version of losetup(8)
+ is used, it can be set to 0, since needed loop devices can be
+ dynamically allocated with the /dev/loop-control interface.

config BLK_DEV_CRYPTOLOOP
tristate "Cryptoloop Support"
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 76c8da7..d9b71ca 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -75,15 +75,21 @@
#include <linux/kthread.h>
#include <linux/splice.h>
#include <linux/sysfs.h>
+#include <linux/miscdevice.h>

#include <asm/uaccess.h>

static LIST_HEAD(loop_devices);
+static DEFINE_IDR(loop_index_idr);
static DEFINE_MUTEX(loop_devices_mutex);

+static int max_loop;
static int max_part;
static int part_shift;

+static int loop_lookup(struct loop_device **lo, int nr);
+static int loop_add(struct loop_device **lo, int nr);
+
/*
* Transfer functions
*/
@@ -722,15 +728,8 @@ static inline int is_loop_device(struct file *file)
static ssize_t loop_attr_show(struct device *dev, char *page,
ssize_t (*callback)(struct loop_device *, char *))
{
- struct loop_device *l, *lo = NULL;
-
- mutex_lock(&loop_devices_mutex);
- list_for_each_entry(l, &loop_devices, lo_list)
- if (disk_to_dev(l->lo_disk) == dev) {
- lo = l;
- break;
- }
- mutex_unlock(&loop_devices_mutex);
+ struct gendisk *disk = dev_to_disk(dev);
+ struct loop_device *lo = disk->private_data;

return lo ? callback(lo, page) : -EIO;
}
@@ -995,7 +994,7 @@ static int loop_clr_fd(struct loop_device *lo, struct block_device *bdev)
if (lo->lo_state != Lo_bound)
return -ENXIO;

- if (lo->lo_refcnt > 1) /* we needed one fd for the ioctl */
+ if (atomic_read(&lo->lo_open_count) > 1) /* we needed one fd for the ioctl */
return -EBUSY;

if (filp == NULL)
@@ -1485,13 +1484,19 @@ 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;
-
- mutex_lock(&lo->lo_ctl_mutex);
- lo->lo_refcnt++;
- mutex_unlock(&lo->lo_ctl_mutex);
+ struct loop_device *lo;
+ int err = 0;

- return 0;
+ mutex_lock(&loop_devices_mutex);
+ lo = bdev->bd_disk->private_data;
+ if (!lo) {
+ err = -ENXIO;
+ goto out;
+ }
+ atomic_inc(&lo->lo_open_count);
+out:
+ mutex_unlock(&loop_devices_mutex);
+ return err;
}

static int lo_release(struct gendisk *disk, fmode_t mode)
@@ -1501,7 +1506,7 @@ static int lo_release(struct gendisk *disk, fmode_t mode)

mutex_lock(&lo->lo_ctl_mutex);

- if (--lo->lo_refcnt)
+ if (!atomic_dec_and_test(&lo->lo_open_count))
goto out;

if (lo->lo_flags & LO_FLAGS_AUTOCLEAR) {
@@ -1536,10 +1541,6 @@ static const struct block_device_operations lo_fops = {
#endif
};

-/*
- * And now the modules code and kernel interface.
- */
-static int max_loop;
module_param(max_loop, int, S_IRUGO);
MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
module_param(max_part, int, S_IRUGO);
@@ -1556,136 +1557,244 @@ int loop_register_transfer(struct loop_func_table *funcs)
xfer_funcs[n] = funcs;
return 0;
}
+EXPORT_SYMBOL(loop_register_transfer);
+
+static int unregister_transfer_cb(int id, void *ptr, void *data)
+{
+ struct loop_device *lo = ptr;
+ struct loop_func_table *xfer = data;
+
+ mutex_lock(&lo->lo_ctl_mutex);
+ if (lo->lo_encryption == xfer)
+ loop_release_xfer(lo);
+ mutex_unlock(&lo->lo_ctl_mutex);
+ return 0;
+}

int loop_unregister_transfer(int number)
{
unsigned int n = number;
- struct loop_device *lo;
struct loop_func_table *xfer;

if (n == 0 || n >= MAX_LO_CRYPT || (xfer = xfer_funcs[n]) == NULL)
return -EINVAL;

xfer_funcs[n] = NULL;
-
- list_for_each_entry(lo, &loop_devices, lo_list) {
- mutex_lock(&lo->lo_ctl_mutex);
-
- if (lo->lo_encryption == xfer)
- loop_release_xfer(lo);
-
- mutex_unlock(&lo->lo_ctl_mutex);
- }
-
+ idr_for_each(&loop_index_idr, &unregister_transfer_cb, xfer);
return 0;
}
-
-EXPORT_SYMBOL(loop_register_transfer);
EXPORT_SYMBOL(loop_unregister_transfer);

-static struct loop_device *loop_alloc(int i)
+static int loop_add(struct loop_device **lo, int nr)
{
- struct loop_device *lo;
+ struct loop_device *l;
struct gendisk *disk;
+ int err;

- lo = kzalloc(sizeof(*lo), GFP_KERNEL);
- if (!lo)
- goto out;
+ l = kzalloc(sizeof(struct loop_device), GFP_KERNEL);
+ if (!l) {
+ err = -ENOMEM;
+ goto err;
+ }

- lo->lo_queue = blk_alloc_queue(GFP_KERNEL);
- if (!lo->lo_queue)
- goto out_free_dev;
-
- disk = lo->lo_disk = alloc_disk(1 << part_shift);
- if (!disk)
- goto out_free_queue;
-
- mutex_init(&lo->lo_ctl_mutex);
- lo->lo_number = i;
- lo->lo_thread = NULL;
- init_waitqueue_head(&lo->lo_event);
- spin_lock_init(&lo->lo_lock);
- disk->major = LOOP_MAJOR;
- disk->first_minor = i << part_shift;
- disk->fops = &lo_fops;
- disk->private_data = lo;
- disk->queue = lo->lo_queue;
- sprintf(disk->disk_name, "loop%d", i);
- return lo;
-
-out_free_queue:
- blk_cleanup_queue(lo->lo_queue);
-out_free_dev:
- kfree(lo);
-out:
- return NULL;
+ err = idr_pre_get(&loop_index_idr, GFP_KERNEL);
+ if (err < 0)
+ goto err_free_dev;
+
+ if (nr >= 0) {
+ int m;
+
+ /* create specific nr in the index */
+ err = idr_get_new_above(&loop_index_idr, l, nr, &m);
+ if (err >= 0 && nr != m) {
+ idr_remove(&loop_index_idr, m);
+ err = -EEXIST;
+ }
+ } else if (nr == -1) {
+ int m;
+
+ /* get next free nr */
+ err = idr_get_new(&loop_index_idr, l, &m);
+ if (err >= 0)
+ nr = m;
+ } else {
+ err = -EINVAL;
+ }
+ if (err < 0)
+ goto err_free_dev;
+
+ l->lo_queue = blk_alloc_queue(GFP_KERNEL);
+ if (!l->lo_queue) {
+ err = -ENOMEM;
+ goto err_free_dev;
+ }
+
+ disk = alloc_disk(1 << part_shift);
+ if (!disk) {
+ err = -ENOMEM;
+ goto err_free_queue;
+ }
+ disk->first_minor = nr << part_shift;
+ disk->major = LOOP_MAJOR;
+ disk->fops = &lo_fops;
+ disk->private_data = l;
+ disk->queue = l->lo_queue;
+ sprintf(disk->disk_name, "loop%d", nr);
+ add_disk(disk);
+ l->lo_disk = disk;
+
+ l->lo_number = nr;
+ mutex_init(&l->lo_ctl_mutex);
+ init_waitqueue_head(&l->lo_event);
+ spin_lock_init(&l->lo_lock);
+ *lo = l;
+
+ return l->lo_number;
+
+err_free_queue:
+ blk_cleanup_queue(l->lo_queue);
+err_free_dev:
+ kfree(l);
+err:
+ return err;
}

-static void loop_free(struct loop_device *lo)
+static int find_free_cb(int id, void *ptr, void *data)
{
- blk_cleanup_queue(lo->lo_queue);
- put_disk(lo->lo_disk);
- list_del(&lo->lo_list);
- kfree(lo);
+ struct loop_device *l = ptr;
+ struct loop_device **lo = data;
+
+ if (l->lo_state == Lo_unbound) {
+ *lo = l;
+ return 1;
+ }
+ return 0;
}

-static struct loop_device *loop_init_one(int i)
+static int loop_lookup(struct loop_device **lo, int nr)
{
- struct loop_device *lo;
+ struct loop_device *l;
+ int ret = -ENODEV;

- list_for_each_entry(lo, &loop_devices, lo_list) {
- if (lo->lo_number == i)
- return lo;
+ if (nr < 0) {
+ int err;
+
+ err = idr_for_each(&loop_index_idr, &find_free_cb, &l);
+ if (err == 1) {
+ *lo = l;
+ ret = l->lo_number;
+ }
+ goto out;
}

- lo = loop_alloc(i);
- if (lo) {
- add_disk(lo->lo_disk);
- list_add_tail(&lo->lo_list, &loop_devices);
+ /* lookup and return a specific nr */
+ l = idr_find(&loop_index_idr, nr);
+ if (l) {
+ *lo = l;
+ ret = l->lo_number;
}
- return lo;
+out:
+ return ret;
}

-static void loop_del_one(struct loop_device *lo)
+static void loop_remove(struct loop_device *lo)
{
del_gendisk(lo->lo_disk);
- loop_free(lo);
+ blk_cleanup_queue(lo->lo_queue);
+ put_disk(lo->lo_disk);
+ kfree(lo);
}

static struct kobject *loop_probe(dev_t dev, int *part, void *data)
{
struct loop_device *lo;
struct kobject *kobj;
+ int err;

mutex_lock(&loop_devices_mutex);
- lo = loop_init_one(MINOR(dev) >> part_shift);
- kobj = lo ? get_disk(lo->lo_disk) : ERR_PTR(-ENOMEM);
+ err = loop_lookup(&lo, MINOR(dev) >> part_shift);
+ if (err < 0)
+ err = loop_add(&lo, MINOR(dev) >> part_shift);
+ if (err < 0)
+ kobj = ERR_PTR(err);
+ else
+ kobj = get_disk(lo->lo_disk);
mutex_unlock(&loop_devices_mutex);

*part = 0;
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_devices_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;
+ if (lo->lo_state != Lo_unbound) {
+ ret = -EBUSY;
+ break;
+ }
+ if (atomic_read(&lo->lo_open_count) > 0) {
+ ret = -EBUSY;
+ break;
+ }
+ lo->lo_disk->private_data = NULL;
+ 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_devices_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 err;
int i, nr;
unsigned long range;
- struct loop_device *lo, *next;
+ struct loop_device *lo;

- /*
- * loop module now has a feature to instantiate underlying device
- * structure on-demand, provided that there is an access dev node.
- * However, this will not work well with user space tool that doesn't
- * know about such "feature". In order to not break any existing
- * tool, we do the following:
- *
- * (1) if max_loop is specified, create that many upfront, and this
- * also becomes a hard limit.
- * (2) if max_loop is not specified, create 8 loop device on module
- * load, user can further extend loop device by create dev node
- * themselves and have kernel automatically instantiate actual
- * device on-demand.
- */
+ err = misc_register(&loop_misc);
+ if (err < 0)
+ return err;

part_shift = 0;
if (max_part > 0) {
@@ -1704,61 +1813,62 @@ static int __init loop_init(void)

if ((1UL << part_shift) > DISK_MAX_PARTS)
return -EINVAL;
-
if (max_loop > 1UL << (MINORBITS - part_shift))
return -EINVAL;

+ /*
+ * If max_loop is specified, create that many upfront. This also
+ * becomes a hard limit. If max_loop is not specified, create
+ * CONFIG_BLK_DEV_LOOP_MIN_COUNT loop devices at init time.
+ * Loop devices can be requested on-demand with the
+ * /dev/loop-control interface, or be instantiated by accessing
+ * a device node.
+ */
if (max_loop) {
nr = max_loop;
range = max_loop << part_shift;
} else {
- nr = 8;
+ nr = CONFIG_BLK_DEV_LOOP_MIN_COUNT;
range = 1UL << MINORBITS;
}

if (register_blkdev(LOOP_MAJOR, "loop"))
return -EIO;
-
- for (i = 0; i < nr; i++) {
- lo = loop_alloc(i);
- if (!lo)
- goto Enomem;
- list_add_tail(&lo->lo_list, &loop_devices);
- }
-
- /* point of no return */
-
- list_for_each_entry(lo, &loop_devices, lo_list)
- add_disk(lo->lo_disk);
-
blk_register_region(MKDEV(LOOP_MAJOR, 0), range,
THIS_MODULE, loop_probe, NULL, NULL);

+ /* pre-create number devices of devices given by config or max_loop */
+ mutex_lock(&loop_devices_mutex);
+ for (i = 0; i < nr; i++)
+ loop_add(&lo, i);
+ mutex_unlock(&loop_devices_mutex);
+
printk(KERN_INFO "loop: module loaded\n");
return 0;
+}

-Enomem:
- printk(KERN_INFO "loop: out of memory\n");
-
- list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
- loop_free(lo);
+static int loop_exit_cb(int id, void *ptr, void *data)
+{
+ struct loop_device *lo = ptr;

- unregister_blkdev(LOOP_MAJOR, "loop");
- return -ENOMEM;
+ loop_remove(lo);
+ return 0;
}

static void __exit loop_exit(void)
{
unsigned long range;
- struct loop_device *lo, *next;

range = max_loop ? max_loop << part_shift : 1UL << MINORBITS;

- list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
- loop_del_one(lo);
+ idr_for_each(&loop_index_idr, &loop_exit_cb, NULL);
+ idr_remove_all(&loop_index_idr);
+ idr_destroy(&loop_index_idr);

blk_unregister_region(MKDEV(LOOP_MAJOR, 0), range);
unregister_blkdev(LOOP_MAJOR, "loop");
+
+ misc_deregister(&loop_misc);
}

module_init(loop_init);
@@ -1770,6 +1880,5 @@ static int __init max_loop_setup(char *str)
max_loop = simple_strtol(str, NULL, 0);
return 1;
}
-
__setup("max_loop=", max_loop_setup);
#endif
diff --git a/include/linux/loop.h b/include/linux/loop.h
index 66c194e..5c6d011 100644
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -30,10 +30,10 @@ struct loop_func_table;

struct loop_device {
int lo_number;
- int lo_refcnt;
+ int lo_flags;
loff_t lo_offset;
loff_t lo_sizelimit;
- int lo_flags;
+ atomic_t lo_open_count;
int (*transfer)(struct loop_device *, int cmd,
struct page *raw_page, unsigned raw_off,
struct page *loop_page, unsigned loop_off,
@@ -64,7 +64,6 @@ struct loop_device {

struct request_queue *lo_queue;
struct gendisk *lo_disk;
- struct list_head lo_list;
};

#endif /* __KERNEL__ */
@@ -161,4 +160,9 @@ 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
--- 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;


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