NFSD: delay unmount source's export after inter-server copy completed.

[ Upstream commit f4e44b393389c77958f7c58bf4415032b4cda15b ]

Currently the source's export is mounted and unmounted on every
inter-server copy operation. This patch is an enhancement to delay
the unmount of the source export for a certain period of time to
eliminate the mount and unmount overhead on subsequent copy operations.

After a copy operation completes, a work entry is added to the
delayed unmount list with an expiration time. This list is serviced
by the laundromat thread to unmount the export of the expired entries.
Each time the export is being used again, its expiration time is
extended and the entry is re-inserted to the tail of the list.

The unmount task and the mount operation of the copy request are
synced to make sure the export is not unmounted while it's being
used.

Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/linux/nfs_ssc.h

@@ -8,6 +8,7 @@
  */
 
 #include <linux/nfs_fs.h>
+#include <linux/sunrpc/svc.h>
 
 extern struct nfs_ssc_client_ops_tbl nfs_ssc_client_tbl;
 
@@ -52,6 +53,19 @@ static inline void nfs42_ssc_close(struct file *filep)
 	if (nfs_ssc_client_tbl.ssc_nfs4_ops)
 		(*nfs_ssc_client_tbl.ssc_nfs4_ops->sco_close)(filep);
 }
+
+struct nfsd4_ssc_umount_item {
+	struct list_head nsui_list;
+	bool nsui_busy;
+	/*
+	 * nsui_refcnt inited to 2, 1 on list and 1 for consumer. Entry
+	 * is removed when refcnt drops to 1 and nsui_expire expires.
+	 */
+	refcount_t nsui_refcnt;
+	unsigned long nsui_expire;
+	struct vfsmount *nsui_vfsmount;
+	char nsui_ipaddr[RPC_MAX_ADDRBUFLEN];
+};
 #endif
 
 /*