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Date:	Thu, 29 Mar 2007 06:16:32 +0200 (MEST)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	Kyle Moffett <mrlinuxman@....com>
cc:	Eric Dumazet <dada1@...mosbay.com>, Tomas M <tomas@...x.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] max_loop limit, t2



On Mar 25 2007 10:40, Tomas M wrote:
>On ??, Jan Engelhardt wrote:
>
>> here's one. Allocates all the fluff dynamically. It does not
>> create any dev nodes by itself, so you need to do it (à la mdadm)
>
> I'm afraid that this would break a lot of things, for example mount
> -o loop will not work anymore unless you create /dev/loop* manually
> first, am I correct? In this case, this is unusable for many as it
> is not backward compatible with old loop.c, am I correct?

So here's another try. Use the max_auto_loop= module parameter to
define how many device nodes should be created (defaults to 8, like
original loop.c) in advance. (More specifically, how many disks you
want uevents have generated.) This is because creating all 1048576
possible loop disks in /dev (tmpfs!!) would be really overkill and
seldom good for memory usage.


On Mar 28 2007 23:54, Kyle Moffett wrote:

> Maybe an rbtree would work better here?  Maximum number of nodes
> traversed to get to the bottom of the tree given 2^(20) loop
> devices is 19 as opposed to the 2^(20) for a linked list.  Also, to
> preserve compatibility with existing userspace loop tools you
> should probably always allocate one extra loop device. Keep a
> "highest used loopdev" number and create the one after that so that
> udev will autocreate a dev node for it.

Yeah I already have a ... hack that creates /dev/loop[0-7] but it
segfaults ^_^ Perhaps someone knows why. The oops trace I get has
kobject_uevent() in it, but I don't think I missed something in the
_init function wrt. uevent generation, did I?

Signed-off-by: Jan Engelhardt <jengelh@....de>

Name: dynamic-loop-jengelh2.diff

Index: linux-2.6.21-rc5/drivers/block/Makefile
===================================================================
--- linux-2.6.21-rc5.orig/drivers/block/Makefile
+++ linux-2.6.21-rc5/drivers/block/Makefile
@@ -29,3 +29,4 @@ obj-$(CONFIG_VIODASD)		+= viodasd.o
 obj-$(CONFIG_BLK_DEV_SX8)	+= sx8.o
 obj-$(CONFIG_BLK_DEV_UB)	+= ub.o
 
+CFLAGS_loop.o += -O0
Index: linux-2.6.21-rc5/drivers/block/loop.c
===================================================================
--- linux-2.6.21-rc5.orig/drivers/block/loop.c
+++ linux-2.6.21-rc5/drivers/block/loop.c
@@ -77,9 +77,9 @@
 
 #include <asm/uaccess.h>
 
-static int max_loop = 8;
-static struct loop_device *loop_dev;
-static struct gendisk **disks;
+static unsigned int max_auto_loop = 8;
+static LIST_HEAD(loop_devices);
+static DEFINE_SPINLOCK(loop_devices_lock);
 
 /*
  * Transfer functions
@@ -183,7 +183,7 @@ figure_loop_size(struct loop_device *lo)
 	if (unlikely((loff_t)x != size))
 		return -EFBIG;
 
-	set_capacity(disks[lo->lo_number], x);
+	set_capacity(lo->lo_disk, x);
 	return 0;					
 }
 
@@ -812,7 +812,7 @@ static int loop_set_fd(struct loop_devic
 	lo->lo_queue->queuedata = lo;
 	lo->lo_queue->unplug_fn = loop_unplug;
 
-	set_capacity(disks[lo->lo_number], size);
+	set_capacity(lo->lo_disk, size);
 	bd_set_size(bdev, size << 9);
 
 	set_blocksize(bdev, lo_blocksize);
@@ -832,7 +832,7 @@ out_clr:
 	lo->lo_device = NULL;
 	lo->lo_backing_file = NULL;
 	lo->lo_flags = 0;
-	set_capacity(disks[lo->lo_number], 0);
+	set_capacity(lo->lo_disk, 0);
 	invalidate_bdev(bdev, 0);
 	bd_set_size(bdev, 0);
 	mapping_set_gfp_mask(mapping, lo->old_gfp_mask);
@@ -918,7 +918,7 @@ static int loop_clr_fd(struct loop_devic
 	memset(lo->lo_crypt_name, 0, LO_NAME_SIZE);
 	memset(lo->lo_file_name, 0, LO_NAME_SIZE);
 	invalidate_bdev(bdev, 0);
-	set_capacity(disks[lo->lo_number], 0);
+	set_capacity(lo->lo_disk, 0);
 	bd_set_size(bdev, 0);
 	mapping_set_gfp_mask(filp->f_mapping, gfp);
 	lo->lo_state = Lo_unbound;
@@ -1357,8 +1357,9 @@ static struct block_device_operations lo
 /*
  * And now the modules code and kernel interface.
  */
-module_param(max_loop, int, 0);
-MODULE_PARM_DESC(max_loop, "Maximum number of loop devices (1-256)");
+module_param(max_auto_loop, uint, S_IRUGO);
+MODULE_PARM_DESC(max_auto_loop, "Maximum number of auto-generated loop device "
+	"nodes (0-1048576)");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
 
@@ -1383,7 +1384,7 @@ int loop_unregister_transfer(int number)
 
 	xfer_funcs[n] = NULL;
 
-	for (lo = &loop_dev[0]; lo < &loop_dev[max_loop]; lo++) {
+	list_for_each_entry(lo, &loop_devices, lo_list) {
 		mutex_lock(&lo->lo_ctl_mutex);
 
 		if (lo->lo_encryption == xfer)
@@ -1398,102 +1399,120 @@ int loop_unregister_transfer(int number)
 EXPORT_SYMBOL(loop_register_transfer);
 EXPORT_SYMBOL(loop_unregister_transfer);
 
-static int __init loop_init(void)
+static struct loop_device *loop_find_dev(unsigned int number)
+{
+	struct loop_device *lo;
+	list_for_each_entry(lo, &loop_devices, lo_list)
+		if (lo->lo_number == number)
+			return lo;
+	return NULL;
+}
+
+static struct loop_device *loop_init_one(unsigned int number)
 {
-	int	i;
+	struct loop_device *lo;
+	struct gendisk *disk;
+
+	lo = kzalloc(sizeof(struct loop_device), GFP_KERNEL);
+	if (lo == NULL)
+		goto out;
 
-	if (max_loop < 1 || max_loop > 256) {
-		printk(KERN_WARNING "loop: invalid max_loop (must be between"
-				    " 1 and 256), using default (8)\n");
-		max_loop = 8;
+	lo->lo_queue = blk_alloc_queue(GFP_KERNEL);
+	if (lo->lo_queue == NULL)
+		goto out_free_dev;
+
+	disk = lo->lo_disk = alloc_disk(1);
+	if (disk == NULL)
+		goto out_free_queue;
+
+	mutex_init(&lo->lo_ctl_mutex);
+	lo->lo_number      = number;
+	lo->lo_thread      = NULL;
+	init_waitqueue_head(&lo->lo_event);
+	spin_lock_init(&lo->lo_lock);
+	disk->major        = LOOP_MAJOR;
+	disk->first_minor  = number;
+	disk->fops         = &lo_fops;
+	disk->private_data = lo;
+	disk->queue        = lo->lo_queue;
+	sprintf(disk->disk_name, "loop%u", number);
+	add_disk(lo->lo_disk);
+
+	spin_lock(&loop_devices_lock);
+	list_add_tail(&lo->lo_list, &loop_devices);
+	spin_unlock(&loop_devices_lock);
+	return lo;
+
+ out_free_queue:
+	blk_cleanup_queue(lo->lo_queue);
+ out_free_dev:
+	kfree(lo);
+ out:
+	return ERR_PTR(-ENOMEM);
+}
+
+static void loop_del_one(struct loop_device *lo)
+{
+	del_gendisk(lo->lo_disk);
+	blk_cleanup_queue(lo->lo_queue);
+	put_disk(lo->lo_disk);
+	list_del(&lo->lo_list);
+	kfree(lo);
+	return;
+}
+
+static struct kobject *loop_probe(dev_t dev, int *part, void *data)
+{
+	unsigned int number = dev & MINORMASK;
+	struct loop_device *lo;
+
+	if ((lo = loop_find_dev(number)) == NULL) {
+		lo = loop_init_one(number);
+		if (IS_ERR(lo))
+			return (void *)lo;
+		*part = 0;
 	}
 
+	return &lo->lo_disk->kobj;
+}
+
+static int __init loop_init(void)
+{
+	struct kobject kobj;
+	int i;
+
 	if (register_blkdev(LOOP_MAJOR, "loop"))
 		return -EIO;
+	blk_register_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS,
+	                    THIS_MODULE, loop_probe, NULL, NULL);
 
-	loop_dev = kmalloc(max_loop * sizeof(struct loop_device), GFP_KERNEL);
-	if (!loop_dev)
-		goto out_mem1;
-	memset(loop_dev, 0, max_loop * sizeof(struct loop_device));
-
-	disks = kmalloc(max_loop * sizeof(struct gendisk *), GFP_KERNEL);
-	if (!disks)
-		goto out_mem2;
-
-	for (i = 0; i < max_loop; i++) {
-		disks[i] = alloc_disk(1);
-		if (!disks[i])
-			goto out_mem3;
-	}
-
-	for (i = 0; i < max_loop; i++) {
-		struct loop_device *lo = &loop_dev[i];
-		struct gendisk *disk = disks[i];
-
-		memset(lo, 0, sizeof(*lo));
-		lo->lo_queue = blk_alloc_queue(GFP_KERNEL);
-		if (!lo->lo_queue)
-			goto out_mem4;
-		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;
-		disk->fops = &lo_fops;
-		sprintf(disk->disk_name, "loop%d", i);
-		disk->private_data = lo;
-		disk->queue = lo->lo_queue;
-	}
-
-	/* We cannot fail after we call this, so another loop!*/
-	for (i = 0; i < max_loop; i++)
-		add_disk(disks[i]);
-	printk(KERN_INFO "loop: loaded (max %d devices)\n", max_loop);
-	return 0;
+	/* Trigger uevents so that /dev nodes get created by udev */
+	for (i = 0; i < max_auto_loop; ++i) {
+		kobject_init(&kobj);
+		if (kobject_set_name(&kobj, "loop%u", i) != 0)
+			/*
+			 * Most likely the Out Of Memory case. Do not try to
+			 * create more objects.
+			 */
+			break;
+		kobject_uevent(&kobj, KOBJ_ADD);
+		kobject_put(&kobj);
+	}
 
-out_mem4:
-	while (i--)
-		blk_cleanup_queue(loop_dev[i].lo_queue);
-	i = max_loop;
-out_mem3:
-	while (i--)
-		put_disk(disks[i]);
-	kfree(disks);
-out_mem2:
-	kfree(loop_dev);
-out_mem1:
-	unregister_blkdev(LOOP_MAJOR, "loop");
-	printk(KERN_ERR "loop: ran out of memory\n");
-	return -ENOMEM;
+	return 0;
 }
 
-static void loop_exit(void)
+static void __exit loop_exit(void)
 {
-	int i;
+	struct loop_device *lo, *next;
 
-	for (i = 0; i < max_loop; i++) {
-		del_gendisk(disks[i]);
-		blk_cleanup_queue(loop_dev[i].lo_queue);
-		put_disk(disks[i]);
-	}
+	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
+		loop_del_one(lo);
+
+	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS);
 	if (unregister_blkdev(LOOP_MAJOR, "loop"))
 		printk(KERN_WARNING "loop: cannot unregister blkdev\n");
-
-	kfree(disks);
-	kfree(loop_dev);
 }
 
 module_init(loop_init);
 module_exit(loop_exit);
-
-#ifndef MODULE
-static int __init max_loop_setup(char *str)
-{
-	max_loop = simple_strtol(str, NULL, 0);
-	return 1;
-}
-
-__setup("max_loop=", max_loop_setup);
-#endif
Index: linux-2.6.21-rc5/include/linux/loop.h
===================================================================
--- linux-2.6.21-rc5.orig/include/linux/loop.h
+++ linux-2.6.21-rc5/include/linux/loop.h
@@ -16,6 +16,7 @@
 #ifdef __KERNEL__
 #include <linux/bio.h>
 #include <linux/blkdev.h>
+#include <linux/list.h>
 #include <linux/spinlock.h>
 #include <linux/mutex.h>
 
@@ -29,7 +30,8 @@ enum {
 struct loop_func_table;
 
 struct loop_device {
-	int		lo_number;
+	struct list_head lo_list;
+	unsigned int	lo_number;
 	int		lo_refcnt;
 	loff_t		lo_offset;
 	loff_t		lo_sizelimit;
@@ -50,6 +52,7 @@ struct loop_device {
 
 	struct file *	lo_backing_file;
 	struct block_device *lo_device;
+	struct gendisk *lo_disk;
 	unsigned	lo_blocksize;
 	void		*key_data; 
 
#<EOF>


Jan
-- 
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