NFSv4.1: Fix umount when filelayout DS is also the MDS

Currently there is a 'chicken and egg' issue when the DS is also the mounted
MDS. The nfs_match_client() reference from nfs4_set_ds_client bumps the
cl_count, the nfs_client is not freed at umount, and nfs4_deviceid_purge_client
is not called to dereference the MDS usage of a deviceid which holds a
reference to the DS nfs_client.  The result is the umount program returns,
but the nfs_client is not freed, and the cl_session hearbeat continues.

The MDS (and all other nfs mounts) lose their last nfs_client reference in
nfs_free_server when the last nfs_server (fsid) is umounted.
The file layout DS lose their last nfs_client reference in destroy_ds
when the last deviceid referencing the data server is put and destroy_ds is
called. This is triggered by a call to nfs4_deviceid_purge_client which
removes references to a pNFS deviceid used by an MDS mount.

The fix is to track how many pnfs enabled filesystems are mounted from
this server, and then to purge the device id cache once that count reaches
zero.

Reported-by: Jorge Mora <Jorge.Mora@netapp.com>
Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/client.c

@@ -207,7 +207,6 @@ error_0:
 static void nfs4_shutdown_session(struct nfs_client *clp)
 {
 	if (nfs4_has_session(clp)) {
-		nfs4_deviceid_purge_client(clp);
 		nfs4_destroy_session(clp->cl_session);
 		nfs4_destroy_clientid(clp);
 	}

fs/nfs/pnfs.c

@@ -80,6 +80,9 @@ unset_pnfs_layoutdriver(struct nfs_server *nfss)
 	if (nfss->pnfs_curr_ld) {
 		if (nfss->pnfs_curr_ld->clear_layoutdriver)
 			nfss->pnfs_curr_ld->clear_layoutdriver(nfss);
+		/* Decrement the MDS count. Purge the deviceid cache if zero */
+		if (atomic_dec_and_test(&nfss->nfs_client->cl_mds_count))
+			nfs4_deviceid_purge_client(nfss->nfs_client);
 		module_put(nfss->pnfs_curr_ld->owner);
 	}
 	nfss->pnfs_curr_ld = NULL;
@@ -127,6 +130,8 @@ set_pnfs_layoutdriver(struct nfs_server *server, const struct nfs_fh *mntfh,
 		module_put(ld_type->owner);
 		goto out_no_driver;
 	}
+	/* Bump the MDS count */
+	atomic_inc(&server->nfs_client->cl_mds_count);
 
 	dprintk("%s: pNFS module for %u set\n", __func__, id);
 	return;

include/linux/nfs_fs_sb.h

@@ -25,6 +25,7 @@ struct nfs41_impl_id;
  */
 struct nfs_client {
 	atomic_t		cl_count;
+	atomic_t		cl_mds_count;
 	int			cl_cons_state;	/* current construction state (-ve: init error) */
 #define NFS_CS_READY		0		/* ready to be used */
 #define NFS_CS_INITING		1		/* busy initialising */