[PATCH 0/6 RFC v2] nfsd: allocate/free session-based DRC slots on demand

[NeilBrown <neilb@suse.de>]

Here is v2 of my series for on-demand allocation and freeing of session DRC slots.

- Now uses an xarray to store slots, and the limit is raised to 2048
- delays retiring a slot until the client has confirmed that it isn't
  using it as described in RFC:

      The replier SHOULD retain the slots it wants to retire until the
      requester sends a request with a highest_slotid less than or equal
      to the replier's new enforced highest_slotid.

- When a retired slot is used, allow the seqid to be the next in sequence
  as required by the RFC:

         Each time a slot is reused, the request MUST specify a sequence
         ID that is one greater than that of the previous request on the
         slot.

  or "1" as (arguably) allowed by the RFC:

         The first time a slot is used, the requester MUST specify a
         sequence ID of one

- current slot allocation is now reported in /proc/fs/nfsd/clients/*/info

This has been tested with highly aggressive shrinker settings:
	nfsd_slot_shrinker->seeks = 0;
	nfsd_slot_shrinker->batch = 2;

and with periodic "echo 3 > drop_caches".  The slot count drops as
expected and then increases again.

NeilBrown