Re: [PATCH v15 04/12] block: add emulation for copy

[Nitesh Shetty]

On Fri, Sep 08, 2023 at 08:06:38AM +0200, Hannes Reinecke wrote:
> On 9/6/23 18:38, Nitesh Shetty wrote:
> > For the devices which does not support copy, copy emulation is added.
> > It is required for in-kernel users like fabrics, where file descriptor is
> > not available and hence they can't use copy_file_range.
> > Copy-emulation is implemented by reading from source into memory and
> > writing to the corresponding destination.
> > Also emulation can be used, if copy offload fails or partially completes.
> > At present in kernel user of emulation is NVMe fabrics.
> > 
> Leave out the last sentence; I really would like to see it enabled for SCSI,
> too (we do have copy offload commands for SCSI ...).
> 
Sure, will do that

> And it raises all the questions which have bogged us down right from the
> start: where is the point in calling copy offload if copy offload is not
> implemented or slower than copying it by hand?
> And how can the caller differentiate whether copy offload bring a benefit to
> him?
> 
> IOW: wouldn't it be better to return -EOPNOTSUPP if copy offload is not
> available?

Present approach treats copy as a background operation and the idea is to
maximize the chances of achieving copy by falling back to emulation.
Having said that, it should be possible to return -EOPNOTSUPP,
in case of offload IO failure or device not supporting offload.
We will update this in next version.

Thank you,
Nitesh Shetty