Re: [PATCH v7 1/1] nvmet: support reservation feature

[Guixin Liu <kanie@linux.alibaba.com>]

> unlike abort, preempt-and-abort needs a semantic guarantee because the 
> consumer
> may rely on this for fencing purposes. So it cannot be supported in 
> "best effort" I think.
>
> A possible implementation would be not to abort as there is no such 
> interface, but
> nvmet may wait for all pending ns IO to complete and disallowing new 
> IO to come in
> (using percpu_ref_kill and percpu_ref_resurrect on ns->ref). This 
> won't work very efficiently
> withALL_REGS reservations though.

Hi Sagi,

I found that if we return an error when the call to 
percpu_ref_tryget_live(&ns->ref) fails,

it might cause hosts that still have permissions to interrupt their IO. 
Additionally,

preempt_and_abort itself holds an ns->ref, we cannot wait the ref to 
become to zero.

The solution I can think of is to add a "per-namespace" percpu_ref to 
the controller for

counting IO issued to a particular namespace by that controller. Then, 
during the execution

of preempt_and_abort, we wait for the count of those preempted and 
unregistered controllers

to drop to zero.

The nsid is user-specified, so we can not use array to store the 
per-namespace percpu_ref,

this will increase lookup overhead if we use xarray.

What do you think Sagi? Or may be we can declare that preempt_and_abort 
is not supported, just

like SPDK does.

Best Regards,

Guixin Liu