nfs: probe for LOCALIO when v3 client reconnects to server

Re-enabling NFSv3 LOCALIO is made more complex (than NFSv4) because v3
is stateless.  As such, the hueristic used to identify a LOCALIO probe
point is more adhoc by nature: if/when NFSv3 client IO begins to
complete again in terms of normal RPC-based NFSv3 server IO, attempt
nfs_local_probe_async().

Care is taken to throttle the frequency of nfs_local_probe_async(),
otherwise there could be a flood of repeat calls to
nfs_local_probe_async().

The throttle is admin controlled using a new module parameter for
nfsv3, e.g.:
  echo 512 > /sys/module/nfsv3/parameters/nfs3_localio_probe_throttle

Probe for NFSv3 LOCALIO every N IO requests (512 in this case). Must
be power-of-2, defaults to 0 (probing disabled).

On systems that expect to use LOCALIO with NFSv3 the admin should
configure the 'nfs3_localio_probe_throttle' module parameter.

This commit backfills module parameter documentation in localio.rst

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

1 files changed, 19 insertions, 1 deletions

diff --git a/Documentation/filesystems/nfs/localio.rst b/Documentation/filesystems/nfs/localio.rst
index 7d2dbf75e96d..79808b37d745 100644
--- a/Documentation/filesystems/nfs/localio.rst
+++ b/Documentation/filesystems/nfs/localio.rst
@@ -291,7 +291,7 @@ align their IO).
 Security
 ========
 
-Localio is only supported when UNIX-style authentication (AUTH_UNIX, aka
+LOCALIO is only supported when UNIX-style authentication (AUTH_UNIX, aka
 AUTH_SYS) is used.
 
 Care is taken to ensure the same NFS security mechanisms are used
@@ -306,6 +306,24 @@ client is afforded this same level of access (albeit in terms of the NFS
 protocol via SUNRPC). No other namespaces (user, mount, etc) have been
 altered or purposely extended from the server to the client.
 
+Module Parameters
+=================
+
+/sys/module/nfs/parameters/localio_enabled (bool)
+controls if LOCALIO is enabled, defaults to Y. If client and server are
+local but 'localio_enabled' is set to N then LOCALIO will not be used.
+
+/sys/module/nfs/parameters/localio_O_DIRECT_semantics (bool)
+controls if O_DIRECT extends down to the underlying filesystem, defaults
+to N. Application IO must be logical blocksize aligned, otherwise
+O_DIRECT will fail.
+
+/sys/module/nfsv3/parameters/nfs3_localio_probe_throttle (uint)
+controls if NFSv3 read and write IOs will trigger (re)enabling of
+LOCALIO every N (nfs3_localio_probe_throttle) IOs, defaults to 0
+(disabled). Must be power-of-2, admin keeps all the pieces if they
+misconfigure (too low a value or non-power-of-2).
+
 Testing
 =======