Re: [PATCHv4 0/6] scsi: use xarray for devices and targets

[Christoph Hellwig <hch@lst.de>]

On Tue, Jun 02, 2020 at 01:33:05PM +0200, Hannes Reinecke wrote:
> Hi all,
> 
> based on the ideas from Doug Gilbert here's now my take on using
> xarrays for devices and targets.
> It revolves around two ideas:
> 
> - The scsi target 'channel' and 'id' numbers are never ever used
>   to the full 32 bit range; channels are well below 10, and no
>   driver is using more than 16 bits for the id. So we can reduce
>   the type of 'channel' and 'id' to 16 bits, and use the 32 bit
>   value 'channel << 16 | id' as the index into the target xarray.
> - Nearly every target only ever uses the first two levels of the
>   4-level SCSI LUN structure, which means that we can use the
>   linearized SCSI LUN id as an index into the xarray.
>   If we ever come across targets utilizing more that 2 levels of
>   the LUN structure we'll allocate the first unused index and have
>   to resort to a less efficient lookup instead of direct indexing.
> 
> With these changes we can implement an efficient lookup mechanism,
> devolving into direct lookup for most cases. It also allows us to
> detect duplicate entries or accidental overwrites of existing elements
> by using xa_cmpxchg().
> And iteration over targets and devices should be as efficient as the
> current, list-based, approach.
> 
> As usual, comments and reviews are welcome.

I see absolutely no argument for what the point of this series.  It adds
more code, and I don't really see any indications for it fixing bugs,
speeding up workloads, or reducing memory usage.