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Subject[PATCH 3/3] loop: add BLK_DEV_LOOP_MIN_COUNT=%i to allow distros 0 pre-allocated loop devices
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Date
From: Kay Sievers <kay.sievers@vrfy.org>
Subject: 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>
---
Documentation/kernel-parameters.txt | 9 ++++++---
drivers/block/Kconfig | 15 +++++++++++++++
drivers/block/loop.c | 27 ++++++++++-----------------
3 files changed, 31 insertions(+), 20 deletions(-)

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1340,9 +1340,12 @@ bytes respectively. Such letter suffixes
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]

--- 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
--- 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);



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