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>

3 files changed, 31 insertions, 20 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4ca93898fbd3..c32851131646 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1340,9 +1340,12 @@ 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>
+        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]
 
diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 717d6e4e18d3..57212c5235e2 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -256,6 +256,21 @@ config BLK_DEV_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
+          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
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 5c9edf944879..3defc52f060c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1793,21 +1793,6 @@ static int __init loop_init(void)
         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 @@ static int __init loop_init(void)
         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 @@ static int __init loop_init(void)
         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);