[LSF/MM/BPF TOPIC] block namespaces

[Hannes Reinecke]

On 6/9/21 8:36 PM, James Bottomley wrote:
> On Thu, 2021-05-27 at 10:01 +0200, Hannes Reinecke wrote:
>> Hi all,
>>
>> I guess it's time to tick off yet another item on my long-term to-do
>> list:
>>
>> Block namespaces
>> ----------------
>>
>> Idea is similar to what network already does: allowing each user
>> namespace to have a different 'view' on the existing block devices.
>> EG if the admin creates a ramdisk in one namespace this device should
>> not be visible to other namespaces.
>> But for me the most important use-case would be qemu; currently the
>> devices need to be set up in the host, even though the host has no
>> business touching it as they really belong to the qemu instance. This
>> is causing quite some irritation eg when this device has LVM or MD
>> metadata and udev is trying to activate it on the host.
> 
> I suppose the first question is "why block only?"  There are several
> existing device namespace proposals which would be more generic.
> 

Well; I'm more of a storage person, and do know the needs and 
shortcomings in that area. Less well so in other areas...

>> Overall plan is to restrict views of '/dev', '/sys/dev/block' and
>> '/sys/block' to only present the devices 'visible' for this
>> namespace.
> 
> We actually already have a devices cgroup that does some of this:
> 
> https://www.kernel.org/doc/Documentation/cgroup-v1/devices.txt
> 
I know. But this essentially is a filter on '/dev' only, and needs to be 
configured. Which makes it very unwieldy to use.
And the contents of sysfs are not modified, so there's a mismatch 
between contents in /dev and /sys.
Which might cause issues with monitoring tools.

> However, visibility isn't the only problem, for direct passthrough
> there's also uevent handling and people have even asked about module
> loading.
> 
I am aware, and that's another reason why device cgroup doesn't cut it.

>>   Initially the drivers would keep their global enumeration, but plan
>> is to make the drivers namespace-aware, too, such that each namespace
>> could have its own driver-specific device enumeration.
> 
> I really wouldn't do this.  Namespace/Cgroup separation should be kept
> as high as possible.  If it leaks into the drivers it will become
> unmaintainable.  Why do you think you need the drivers to be aware?  If
> it's just enumeration, that should all be doable with the visibility
> driver unless you want to do things like compact numbering?
> 
Which is precisely why I mentioned device modifications.
On a generic level we can influence the visibility of devices in 
relation to namespaces, we cannot influence the devices themselves.
This will lead to namespaces seeing disjunct device numbers (ie 8:0 and 
8:8 on ns 1, 8:4 on ns 2). Not that I think that will be an issue, but 
certainly a change in behaviour.

>> Goal of this topic is to get a consensus on whether block namespaces
>> are a feature which would find interest, and also to discuss some
>> design details here:
>> - Only in certain cases can a namespace be assigned (eg by calling
>> 'modprobe', starting iscsiadm, or calling nvme-cli); how do we handle
>> devices for which no namespace can be identified?
>> - Shall we allow for different device enumeration per namespace?
>> - Into which level should we go with hiding sysfs structures?
>>    Is blanking out the higher-level interfaces in /dev and /sys/block
>>    enough?
> 
> First question is does the device cgroup do enough for you and if not
> what's missing?
> 
See above. sysfs modifications and uevent filtering are missing.
This infrastructure for that is already in place thanks to network 
namespaces, we 'just' need to make use of it.
Additional drawback is the manual configuration of device-cgroup.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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