loop: add BLK_DEV_LOOP_MIN_COUNT=%i to allow distros 0 pre-allocated loop devices

Instead of unconditionally creating a fixed number of dead loop devices which need to be investigated by storage handling services, even when they are never used, we allow distros start with 0 loop devices and have losetup(8) and similar switch to the dynamic /dev/loop-control interface instead of searching /dev/loop%i for free devices. Signed-off-by: Kay Sievers <kay.sievers@vrfy.org> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

loop: add BLK_DEV_LOOP_MIN_COUNT=%i to allow distros 0 pre-allocated loop devices
Instead of unconditionally creating a fixed number of dead loop devices which need to be investigated by storage handling services, even when they are never used, we allow distros start with 0 loop devices and have losetup(8) and similar switch to the dynamic /dev/loop-control interface instead of searching /dev/loop%i for free devices. Signed-off-by: Kay Sievers <kay.sievers@vrfy.org> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Kay Sievers · Jens Axboe
1 parent 770fe30a46
Showing 3 changed files with 31 additions and 20 deletions Side-by-side Diff
Documentation/kernel-parameters.txt
drivers/block/Kconfig
drivers/block/loop.c
@@ -1340,9 +1340,12 @@
 			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>
+	max_loop=	[LOOP] The number of loop block devices that get
+	(loop.max_loop)	unconditionally pre-created at init time. The default
+			number is configured by BLK_DEV_LOOP_MIN_COUNT. Instead
+			of statically allocating a predefined number, loop
+			devices can be requested on-demand with the
+			/dev/loop-control interface.
  
 	mcatest=	[IA-64]
  
@@ -256,6 +256,21 @@
  
 	  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
+	  Static 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"
 	select CRYPTO
@@ -1793,21 +1793,6 @@
 	struct loop_device *lo;
 	int err;
  
-	/*
-	 * 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;
  
@@ -1833,11 +1818,19 @@
 	if (max_loop > 1UL << (MINORBITS - part_shift))
 		return -EINVAL;
  
+	/*
+	 * If max_loop is specified, create that many devices upfront.
+	 * This also becomes a hard limit. If max_loop is not specified,
+	 * create CONFIG_BLK_DEV_LOOP_MIN_COUNT loop devices at module
+	 * init time. Loop devices can be requested on-demand with the
+	 * /dev/loop-control interface, or be instantiated by accessing
+	 * a 'dead' 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;
 	}
  
@@ -1847,7 +1840,7 @@
 	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 */
+	/* pre-create number of devices given by config or max_loop */
 	mutex_lock(&loop_index_mutex);
 	for (i = 0; i < nr; i++)
 		loop_add(&lo, i);
...	...	@@ -1340,9 +1340,12 @@
1340	1340	it is equivalent to "nosmp", which also disables
1341	1341	the IO APIC.
1342	1342
1343		- max_loop= [LOOP] Maximum number of loopback devices that can
1344		- be mounted
1345		- Format: <1-256>
	1343	+ max_loop= [LOOP] The number of loop block devices that get
	1344	+ (loop.max_loop) unconditionally pre-created at init time. The default
	1345	+ number is configured by BLK_DEV_LOOP_MIN_COUNT. Instead
	1346	+ of statically allocating a predefined number, loop
	1347	+ devices can be requested on-demand with the
	1348	+ /dev/loop-control interface.
1346	1349
1347	1350	mcatest= [IA-64]
1348	1351
...	...	@@ -256,6 +256,21 @@
256	256
257	257	Most users will answer N here.
258	258
	259	+config BLK_DEV_LOOP_MIN_COUNT
	260	+ int "Number of loop devices to pre-create at init time"
	261	+ depends on BLK_DEV_LOOP
	262	+ default 8
	263	+ help
	264	+ Static number of loop devices to be unconditionally pre-created
	265	+ at init time.
	266	+
	267	+ This default value can be overwritten on the kernel command
	268	+ line or with module-parameter loop.max_loop.
	269	+
	270	+ The historic default is 8. If a late 2011 version of losetup(8)
	271	+ is used, it can be set to 0, since needed loop devices can be
	272	+ dynamically allocated with the /dev/loop-control interface.
	273	+
259	274	config BLK_DEV_CRYPTOLOOP
260	275	tristate "Cryptoloop Support"
261	276	select CRYPTO
...	...	@@ -1793,21 +1793,6 @@
1793	1793	struct loop_device *lo;
1794	1794	int err;
1795	1795
1796		- /*
1797		- * loop module now has a feature to instantiate underlying device
1798		- * structure on-demand, provided that there is an access dev node.
1799		- * However, this will not work well with user space tool that doesn't
1800		- * know about such "feature". In order to not break any existing
1801		- * tool, we do the following:
1802		- *
1803		- * (1) if max_loop is specified, create that many upfront, and this
1804		- * also becomes a hard limit.
1805		- * (2) if max_loop is not specified, create 8 loop device on module
1806		- * load, user can further extend loop device by create dev node
1807		- * themselves and have kernel automatically instantiate actual
1808		- * device on-demand.
1809		- */
1810		-
1811	1796	err = misc_register(&loop_misc);
1812	1797	if (err < 0)
1813	1798	return err;
1814	1799
...	...	@@ -1833,11 +1818,19 @@
1833	1818	if (max_loop > 1UL << (MINORBITS - part_shift))
1834	1819	return -EINVAL;
1835	1820
	1821	+ /*
	1822	+ * If max_loop is specified, create that many devices upfront.
	1823	+ * This also becomes a hard limit. If max_loop is not specified,
	1824	+ * create CONFIG_BLK_DEV_LOOP_MIN_COUNT loop devices at module
	1825	+ * init time. Loop devices can be requested on-demand with the
	1826	+ * /dev/loop-control interface, or be instantiated by accessing
	1827	+ * a 'dead' device node.
	1828	+ */
1836	1829	if (max_loop) {
1837	1830	nr = max_loop;
1838	1831	range = max_loop << part_shift;
1839	1832	} else {
1840		- nr = 8;
	1833	+ nr = CONFIG_BLK_DEV_LOOP_MIN_COUNT;
1841	1834	range = 1UL << MINORBITS;
1842	1835	}
1843	1836
...	...	@@ -1847,7 +1840,7 @@
1847	1840	blk_register_region(MKDEV(LOOP_MAJOR, 0), range,
1848	1841	THIS_MODULE, loop_probe, NULL, NULL);
1849	1842
1850		- /* pre-create number devices of devices given by config or max_loop */
	1843	+ /* pre-create number of devices given by config or max_loop */
1851	1844	mutex_lock(&loop_index_mutex);
1852	1845	for (i = 0; i < nr; i++)
1853	1846	loop_add(&lo, i);