Bug 780668

Summary: RN: loop block devices: Only /dev/loop0 usable if max_part parameter is used
Product: [openSUSE] openSUSE 12.2 Reporter: Forgotten User 7XRNh5WElB <forgotten_7XRNh5WElB>
Component: Release NotesAssignee: Karl Eichwalder <ke>
Status: RESOLVED FIXED QA Contact: Stephan Kulow <coolo>
Severity: Normal    
Priority: P3 - Medium CC: jeffm
Version: Final   
Target Milestone: ---   
Hardware: x86-64   
OS: openSUSE 12.2   
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Found By: --- Services Priority:
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Bug Depends on:    
Bug Blocks: 784364    

Description Forgotten User 7XRNh5WElB 2012-09-16 18:09:07 UTC 
User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20100101 Firefox/15.0

If the loop kernel module gets loaded with a max_part parameter, only /dev/loop0 is usable, other loop devices cannot be attached to files.

Reproducible: Always

Steps to Reproduce:
Make sure the loop module is not loaded already.  Then:
$ modprobe loop max_part=8
$ dd if=/dev/null of=/tmp/loopimg seek=1M
$ losetup /dev/loop0 /tmp/loopimg  # <-- OK
$ losetup /dev/loop1 /tmp/loopimg  # <-- Fails
Actual Results:  
failed to setup loop device: No such device or address

Expected Results:  
silent success

I tested with a full openSUSE 12.2 installation from DVD (inside VirtualBox, hosted by 12.1, on x86_64).

I don't know what the error message "No such device or address" refers to.  The file and the (non-attached) loop device node both do exist.

This is a regression, max_part didn't cause such trouble with openSUSE 12.1.

I filed this under "kernel" since the max_part parameter seems to cause this bug, but it might be a bug in the loop userspace tool as well.
Comment 1 Jeff Mahoney 2012-09-27 06:00:08 UTC 
This isn't a bug but rather a change of how things work in the 3.4 kernel.

There are two ways to have partitions for loopback devices. The first is with max_part and the second is with the -P parameter to losetup.

They behave slightly differently since -P will dynamically allocate minor numbers for each device (including adding/removing them on the fly with blockdev --rereadpt). Using the max_part parameter causes each loop device to allocate that many minor numbers for each device.

So when you use max_part = 8 and don't change max_loop, which defaults to 8, you're using all of the allocated minor numbers with the first device.

The solution is either to use -P or to _also_ use max_loop.

This should probably have a release note issued.
Comment 2 Forgotten User 7XRNh5WElB 2012-09-27 19:44:05 UTC 
Aha, things are working as expected now.
Thanks, Jeff, for your detailed explanation,
and sorry for reporting a non-bug.
-- Yarny
Comment 3 Jeff Mahoney 2012-09-27 19:45:05 UTC 
You reported a bug -- we didn't document the change. :)
Comment 4 Karl Eichwalder 2012-10-10 11:38:19 UTC 
Here is my proposal for the release notes (broadly based on your input; is there a readme in the Kernel source docs that I can reference?):



Specifying Partitions for Loopback Devices

With Kernel 3.4 there are two ways to have partitions for loopback devices. The first is with max_part and the second is with the -P parameter to <command>losetup</command>.

They behave slightly differently since -P will dynamically allocate minor
numbers for each device (including adding or removing them on the fly with
<command>blockdev --rereadpt</command>). Using the max_part parameter causes each loop device to allocate that many minor numbers for each device.

So when you use max_part=8 and do not change max_loop, which defaults to 8,
you are using all of the allocated minor numbers with the first device.

The solution is either to use -P or to <emphasis>also</emphasis> use max_loop.
Comment 5 Karl Eichwalder 2012-10-15 11:59:56 UTC 
Fixed in SVN.
Comment 6 Swamp Workflow Management 2012-10-29 12:09:09 UTC 
openSUSE-RU-2012:1407-1: An update that has four recommended fixes can now be installed.

Category: recommended (low)
Bug References: 780668,784291,784364,784757
CVE References: 
Sources used:
openSUSE 12.2 (src):    release-notes-openSUSE-12.2.9-1.12.1