Re: + loop-preallocate-eight-loop-devices.patch added to -mm tree

["Ken Chen" <kenchen@google.com>]

On 5/21/07, Ken Chen <kenchen@google.com> wrote:
> On 5/21/07, Al Viro <viro@ftp.linux.org.uk> wrote:
> > No, it doesn't.  Really.  It's easy to split; untested incremental to your
> > patch follows:
> >
> >         for (i = 0; i < nr; i++) {
> > -               if (!loop_init_one(i))
> > -                       goto err;
> > +               lo = loop_alloc(i);
> > +               if (!lo)
> > +                       goto Enomem;
> > +               list_add_tail(&lo->lo_list, &loop_devices);
> >         }
>
> ah, yes, use the loop_device list_head to link all the pending devices.
>
>
> > +       /* point of no return */
> > +
> > +       list_for_each_entry(lo, &loop_devices, lo_list)
> > +               add_disk(lo->lo_disk);
> > +
> > +       blk_register_region(MKDEV(LOOP_MAJOR, 0), range,
> > +                                 THIS_MODULE, loop_probe, NULL, NULL);
> > +
> >         printk(KERN_INFO "loop: module loaded\n");
> >         return 0;
> > -err:
> > -       loop_exit();
> > +
> > +Enomem:
> >         printk(KERN_INFO "loop: out of memory\n");
> > +
> > +       while(!list_empty(&loop_devices)) {
> > +               lo = list_entry(loop_devices.next, struct loop_device, lo_list);
> > +               loop_del_one(lo);
> > +       }
> > +
> > +       unregister_blkdev(LOOP_MAJOR, "loop");
> >         return -ENOMEM;
> >  }
>
> I suppose the loop_del_one call in Enomem label needs to be split up
> too since in the error path, it hasn't done add_disk() yet?


tested, like this?

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 5526ead..0ed5470 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1354,7 +1354,7 @@ #endif
  */
 static int max_loop;
 module_param(max_loop, int, 0);
-MODULE_PARM_DESC(max_loop, "obsolete, loop device is created on-demand");
+MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);

@@ -1394,16 +1394,11 @@ int loop_unregister_transfer
 EXPORT_SYMBOL(loop_register_transfer);
 EXPORT_SYMBOL(loop_unregister_transfer);

-static struct loop_device *loop_init_one(int i)
+static struct loop_device *loop_alloc(int i)
 {
 	struct loop_device *lo;
 	struct gendisk *disk;

-	list_for_each_entry(lo, &loop_devices, lo_list) {
-		if (lo->lo_number == i)
-			return lo;
-	}
-
 	lo = kzalloc(sizeof(*lo), GFP_KERNEL);
 	if (!lo)
 		goto out;
@@ -1427,8 +1422,6 @@ static struct loop_device *loop_init_one
 	disk->private_data	= lo;
 	disk->queue		= lo->lo_queue;
 	sprintf(disk->disk_name, "loop%d", i);
-	add_disk(disk);
-	list_add_tail(&lo->lo_list, &loop_devices);
 	return lo;

 out_free_queue:
@@ -1439,15 +1432,37 @@ out:
 	return NULL;
 }

-static void loop_del_one(struct loop_device *lo)
+static void loop_free(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);
 }

+static struct loop_device *loop_init_one(int i)
+{
+	struct loop_device *lo;
+
+	list_for_each_entry(lo, &loop_devices, lo_list) {
+		if (lo->lo_number == i)
+			return lo;
+	}
+
+	lo = loop_alloc(i);
+	if (lo) {
+		add_disk(lo->lo_disk);
+		list_add_tail(&lo->lo_list, &loop_devices);
+	}
+	return lo;
+}
+
+static void loop_del_one(struct loop_device *lo)
+{
+	del_gendisk(lo->lo_disk);
+	loop_free(lo);
+}
+
 static struct kobject *loop_probe(dev_t dev, int *part, void *data)
 {
 	struct loop_device *lo;
@@ -1464,28 +1479,77 @@ static struct kobject *loop_probe

 static int __init loop_init(void)
 {
-	if (register_blkdev(LOOP_MAJOR, "loop"))
-		return -EIO;
-	blk_register_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS,
-				  THIS_MODULE, loop_probe, NULL, NULL);
+	int i, nr;
+	unsigned long range;
+	struct loop_device *lo, *next;
+
+	/*
+	 * 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.
+	 */
+	if (max_loop > 1UL << MINORBITS)
+		return -EINVAL;

 	if (max_loop) {
-		printk(KERN_INFO "loop: the max_loop option is obsolete "
-				 "and will be removed in March 2008\n");
+		nr = max_loop;
+		range = max_loop;
+	} else {
+		nr = 8;
+		range = 1UL << MINORBITS;
+	}
+
+	if (register_blkdev(LOOP_MAJOR, "loop"))
+		return -EIO;

+	for (i = 0; i < nr; i++) {
+		lo = loop_alloc(i);
+		if (!lo)
+			goto Enomem;
+		list_add_tail(&lo->lo_list, &loop_devices);
 	}
+
+	/* point of no return */
+
+	list_for_each_entry(lo, &loop_devices, lo_list)
+		add_disk(lo->lo_disk);
+
+	blk_register_region(MKDEV(LOOP_MAJOR, 0), range,
+				  THIS_MODULE, loop_probe, NULL, NULL);
+
 	printk(KERN_INFO "loop: module loaded\n");
 	return 0;
+
+Enomem:
+	printk(KERN_INFO "loop: out of memory\n");
+
+	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
+		loop_free(lo);
+
+	unregister_blkdev(LOOP_MAJOR, "loop");
+	return -ENOMEM;
 }

 static void __exit loop_exit(void)
 {
+	unsigned long range;
 	struct loop_device *lo, *next;

+	range = max_loop ? max_loop :  1UL << MINORBITS;
+
 	list_for_each_entry_safe(lo, next, &loop_devices, lo_list)
 		loop_del_one(lo);

-	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), 1UL << MINORBITS);
+	blk_unregister_region(MKDEV(LOOP_MAJOR, 0), range);
 	if (unregister_blkdev(LOOP_MAJOR, "loop"))
 		printk(KERN_WARNING "loop: cannot unregister blkdev\n");
 }