Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands

[Hannes Reinecke <hare@suse.de>]

On 7/13/22 13:00, Sagi Grimberg wrote:
> 
>>> Maybe the solution is to just not expose a /dev/ng for the mpath device
>>> node, but only for bottom namespaces. Then it would be completely
>>> equivalent to scsi-generic devices.
>>>
>>> It just creates an unexpected mix of semantics of best-effort
>>> multipathing with just path selection, but no requeue/failover...
>>
>> Which is exactly the same semanics as SG_IO on the dm-mpath nodes.
> 
> I view uring passthru somewhat as a different thing than sending SG_IO
> ioctls to dm-mpath. But it can be argued otherwise.
> 
> BTW, the only consumer of it that I'm aware of commented that he
> expects dm-mpath to retry SG_IO when dm-mpath retry for SG_IO submission
> was attempted (https://www.spinics.net/lists/dm-devel/msg46924.html).
> 
>  From Paolo:
> "The problem is that userspace does not have a way to direct the command 
> to a different path in the resubmission. It may not even have permission 
> to issue DM_TABLE_STATUS, or to access the /dev nodes for the underlying 
> paths, so without Martin's patches SG_IO on dm-mpath is basically 
> unreliable by design."
> 
> I didn't manage to track down any followup after that email though...
> 
I did; 'twas me who was involved in the initial customer issue leading 
up to that.

Amongst all the other issue we've found the prime problem with SG_IO is 
that it needs to be directed to the 'active' path.
For the device-mapper has a distinct callout (dm_prepare_ioctl), which 
essentially returns the current active path device. And then the 
device-mapper core issues the command on that active path.

All nice and good, _unless_ that command triggers an error.
Normally it'd be intercepted by the dm-multipath end_io handler, and 
would set the path to offline.
But as ioctls do not use the normal I/O path the end_io handler is never 
called, and further SG_IO calls are happily routed down the failed path.

And the customer had to use SG_IO (or, in qemu-speak, LUN passthrough) 
as his application/filesystem makes heavy use of persistent reservations.

So yeah, nvme-multipathing should be okay here, as io_uring and normal 
I/O are using the same code paths, hence the above scenario really won't 
occur.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
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