RFC possible NFSv2/v3 lock recovery enhancements

For the purpose of discussion, here are 11 WIP patches. These have seen very little testing, and are presented here as a proof of concept only.

There are three main goals of this work:

1) Provide an API for an NFSv2/v3 server administrator to release stale locks

2) Provide an alternate API that doesn't depend on the existance of a particular network loopback interface to allow statd to perform NLM callbacks (ie statd won't have to care whether to use IPv4 or IPv6 loopback to make NLM downcalls)

3) Allow clients to send correct caller_name strings in NLMPROC_LOCK requests.

1 and 2 are met by implementing a set of directories under /sys that expose the kernel's nlm_host and nsm_handle cache. Each entry in the nsm_handle cache has a reboot attribute that would allow a user space process to trigger reboot recovery on the remote peer represented by that entry.

3 is met by adding a new NFSv2/v3 mount option, similar to clientaddr= for NFSv4 mounts, that allows the mount command to specify the caller_name string to use for lock requests on this mount point.

There is still a chicken-and-egg type problem for statd. These new /sys files are owned and readable by only root. The reboot attribute is owned and writable by only root. statd runs as "rpcuser" usually, so unless something special is done, it wouldn't be able to use a /sys file to trigger reboot recovery.

A possible way to address this is to add support for AF_LOCAL to the kernel. The kernel can create an RPC socket that statd can use to perform downcalls. AF_LOCAL can also be used in other cases, like when the kernel has to contact rpcbind, so kernel AF_LOCAL support wouldn't be a single-use type of thing.

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Chuck Lever (11):
lockd: Allow mount option to specify caller_name
lockd: use sm_my_name for nsm_handle lookups
lockd: Keep my_name in nsm_handle
lockd: Add "reboot" attribute to nsm_handles
lockd: Refactor nlm_host_rebooted()
lockd: Add /sys/fs/lockd/nsm_handle/*
lockd: Add /sys/fs/lockd/mon
lockd: Add attributes to /sys/fs/lockd/hosts/*/
lockd: Add /sys/fs/lockd/hosts/*
lockd: Add /sys/fs/lockd/hosts/
lockd: Add /sys/fs/lockd

 fs/lockd/clntlock.c         |    3 
 fs/lockd/clntproc.c         |    6 -
 fs/lockd/host.c             |  319 +++++++++++++++++++++++++++++++++++++++++--
 fs/lockd/mon.c              |  253 ++++++++++++++++++++++++++++++++--
 fs/lockd/svc.c              |   31 ++++
 fs/lockd/svclock.c          |    2 
 fs/nfs/client.c             |   11 +
 fs/nfs/internal.h           |    1 
 fs/nfs/super.c              |    9 +
 include/linux/lockd/bind.h  |    1 
 include/linux/lockd/lockd.h |   19 ++-
 include/linux/nfs_fs_sb.h   |    5 +
 12 files changed, 620 insertions(+), 40 deletions(-)

Chuck Lever