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misc: Increase available dyanmic minors

PATCH In review Keith Busch 2 participants 3 messages Dec 8, 2014 – Dec 9, 2014

Subsystems (2)
Char and misc drivers NVMe
lore
Share & tools
lore

Keith Busch

author
#0 lore raw

PATCH 1/1 misc: Increase available dyanmic minors

This increases the number of available miscellaneous character device dynamic minors from 63 to the max minor, 1M.

Dynamic minor previously started at 63 and went down to zero. That's not enough in some situations, and also eventually creates a collision with 'psaux' misc device. This patch starts minors at the last defined misc minor (255) and works up to the max possible.

Signed-off-by Keith Busch <keith.busch at intel.com>
---
 drivers/char/misc.c |   23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)
drivers/char/misc.c +9 -14
index ffa97d2..229dba5 100644
@@ -59,8 +59,7 @@ static DEFINE_MUTEX(misc_mtx);
 /*
  * Assigned numbers, used for dynamic minors
  */
-#define DYNAMIC_MINORS 64 /* like dynamic majors */
-static DECLARE_BITMAP(misc_minors, DYNAMIC_MINORS);
+static DEFINE_IDA(misc_minors_ida);
 
 #ifdef CONFIG_PROC_FS
 static void *misc_seq_start(struct seq_file *seq, loff_t *pos)

@@ -183,15 +182,14 @@ int misc_register(struct miscdevice * misc)
 	INIT_LIST_HEAD(&misc->list);
 
 	mutex_lock(&misc_mtx);
-
 	if (misc->minor == MISC_DYNAMIC_MINOR) {
-		int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);
-		if (i >= DYNAMIC_MINORS) {
+		int i = ida_simple_get(&misc_minors_ida, MISC_DYNAMIC_MINOR,
+						MINORMASK, GFP_KERNEL);
+		if (i < 0) {
 			err = -EBUSY;
 			goto out;
 		}
-		misc->minor = DYNAMIC_MINORS - i - 1;
-		set_bit(i, misc_minors);
+		misc->minor = i;
 	} else {
 		struct miscdevice *c;
 

@@ -208,9 +206,8 @@ int misc_register(struct miscdevice * misc)
 	misc->this_device = device_create(misc_class, misc->parent, dev,
 					  misc, "%s", misc->name);
 	if (IS_ERR(misc->this_device)) {
-		int i = DYNAMIC_MINORS - misc->minor - 1;
-		if (i < DYNAMIC_MINORS && i >= 0)
-			clear_bit(i, misc_minors);
+		if (misc->minor >= MISC_DYNAMIC_MINOR)
+			ida_simple_remove(&misc_minors_ida, misc->minor);
 		err = PTR_ERR(misc->this_device);
 		goto out;
 	}

@@ -237,16 +234,14 @@ int misc_register(struct miscdevice * misc)
 
 int misc_deregister(struct miscdevice *misc)
 {
-	int i = DYNAMIC_MINORS - misc->minor - 1;
-
 	if (WARN_ON(list_empty(&misc->list)))
 		return -EINVAL;
 
 	mutex_lock(&misc_mtx);
 	list_del(&misc->list);
 	device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));
-	if (i < DYNAMIC_MINORS && i >= 0)
-		clear_bit(i, misc_minors);
+	if (misc->minor >= MISC_DYNAMIC_MINOR)
+		ida_simple_remove(&misc_minors_ida, misc->minor);
 	mutex_unlock(&misc_mtx);
 	return 0;
 }
Signature · 1 line
1.7.10.4
maintainer
#1 lore raw
Quoted 9 lines · On Monday 08 December 2014 16:01:50 Keith Busch wrote:

This increases the number of available miscellaneous character device dynamic minors from 63 to the max minor, 1M.

Dynamic minor previously started at 63 and went down to zero. That's not enough in some situations, and also eventually creates a collision with 'psaux' misc device. This patch starts minors at the last defined misc minor (255) and works up to the max possible.

Signed-off-by Keith Busch <keith.busch at intel.com>

I guess this will break support for ancient user space tools, and possibly also old file systems that do not support more than 8-bit minor numbers. I would assume that it's ok, but you definitely have to mention in the changelog that things might break and how you have concluded that this is safe enough.

If you cannot come up with a good reasoning, it might be better to combine both and use up the traditional dynamic minor numbers before using the >255 range.

Arnd

author
#2 lore raw
Quoted 9 lines · Arnd Bergmann
Quoted 8 lines · On Monday 08 December 2014 16:01:50 Keith Busch wrote:

This increases the number of available miscellaneous character device dynamic minors from 63 to the max minor, 1M.

Dynamic minor previously started at 63 and went down to zero. That's not enough in some situations, and also eventually creates a collision with 'psaux' misc device. This patch starts minors at the last defined misc minor (255) and works up to the max possible.

Quoted 7 lines
Quoted 1 line
Signed-off-by Keith Busch <keith.busch at intel.com>

I guess this will break support for ancient user space tools, and possibly also old file systems that do not support more than 8-bit minor numbers. I would assume that it's ok, but you definitely have to mention in the changelog that things might break and how you have concluded that this is safe enough.

Sure, I can call this out in the change log. I hadn't considered file systems on a character device. Tooling as well.

If a character device is so tightly coupled to such legacy tools, maybe they should register a static minor, and this patch makes 63 additional ones available. Does that sound reasonable?

Quoted 3 lines

If you cannot come up with a good reasoning, it might be better to combine both and use up the traditional dynamic minor numbers before using the >255 range.

A problem with the existing method is that anyone can use a number in the range it's dynamically allocating from. Once you encounter a collision, all following dynamic misc_device registration will fail. Most of the defined minors in that range are marked unused, but PSMOUSE_MINOR is still used.

If we continue using the traditional dynamic allocator, the component registering as dynamic races against the one registering with its static, so that doesn't sound right.