NFS: Avoid quadratic search when freeing delegations.

There are three places that walk all delegation for an nfs_client and
restart whenever they find something interesting - potentially
resulting in a quadratic search:  If there are 10,000 uninteresting
delegations followed by 10,000 interesting one, then the code
skips over 100,000,000 delegations, which can take a noticeable amount
of time.

Of these nfs_delegation_reap_unclaimed() and
nfs_reap_expired_delegations() are only called during unusual events:
a server reboots or reports expired delegations, probably due to a
network partition.  Optimizing these is not particularly important.

The third, nfs_client_return_marked_delegations(), is called
periodically via nfs_expire_unreferenced_delegations().  It could
cause periodic problems on a busy server.

New delegations are added to the end of the list, so if there are
10,000 open files with delegations, and 10,000 more recently opened files
that received delegations but are now closed, then
nfs_client_return_marked_delegations() can take seconds to skip over
the 10,000 open files 10,000 times.  That is a waste of time.

The avoid this waste a place-holder (an inode) is kept when locks are
dropped, so that the place can usually be found again after taking
rcu_readlock().  This place holder ensure that we find the right
starting point in the list of nfs_servers, and makes is probable that
we find the right starting point in the list of delegations.
We might need to occasionally restart at the head of that list.

It might be possible that the place_holder inode could lose its
delegation separately, and then get a new one using the same (freed
and then reallocated) 'struct nfs_delegation'.  Were this to happen,
the new delegation would be at the end of the list and we would miss
returning some other delegations.  This would have the effect of
unnecessarily delaying the return of some unused delegations until the
next time this function is called - typically 90 seconds later.  As
this is not a correctness issue and is vanishingly unlikely to happen,
it does not seem worth addressing.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/delegation.c

@@ -483,28 +483,73 @@ out:
 int nfs_client_return_marked_delegations(struct nfs_client *clp)
 {
 	struct nfs_delegation *delegation;
+	struct nfs_delegation *prev;
 	struct nfs_server *server;
 	struct inode *inode;
+	struct inode *place_holder = NULL;
+	struct nfs_delegation *place_holder_deleg = NULL;
 	int err = 0;
 
 restart:
+	/*
+	 * To avoid quadratic looping we hold a reference
+	 * to an inode place_holder.  Each time we restart, we
+	 * list nfs_servers from the server of that inode, and
+	 * delegation in the server from the delegations of that
+	 * inode.
+	 * prev is an RCU-protected pointer to a delegation which
+	 * wasn't marked for return and might be a good choice for
+	 * the next place_holder.
+	 */
 	rcu_read_lock();
-	list_for_each_entry_rcu(server, &clp->cl_superblocks, client_link) {
-		list_for_each_entry_rcu(delegation, &server->delegations,
-								super_list) {
-			if (!nfs_delegation_need_return(delegation))
+	prev = NULL;
+	if (place_holder)
+		server = NFS_SERVER(place_holder);
+	else
+		server = list_entry_rcu(clp->cl_superblocks.next,
+					struct nfs_server, client_link);
+	list_for_each_entry_from_rcu(server, &clp->cl_superblocks, client_link) {
+		delegation = NULL;
+		if (place_holder && server == NFS_SERVER(place_holder))
+			delegation = rcu_dereference(NFS_I(place_holder)->delegation);
+		if (!delegation || delegation != place_holder_deleg)
+			delegation = list_entry_rcu(server->delegations.next,
+						    struct nfs_delegation, super_list);
+		list_for_each_entry_from_rcu(delegation, &server->delegations, super_list) {
+			struct inode *to_put = NULL;
+
+			if (!nfs_delegation_need_return(delegation)) {
+				prev = delegation;
 				continue;
+			}
 			if (!nfs_sb_active(server->super))
 				break; /* continue in outer loop */
+
+			if (prev) {
+				struct inode *tmp;
+
+				tmp = nfs_delegation_grab_inode(prev);
+				if (tmp) {
+					to_put = place_holder;
+					place_holder = tmp;
+					place_holder_deleg = prev;
+				}
+			}
+
 			inode = nfs_delegation_grab_inode(delegation);
 			if (inode == NULL) {
 				rcu_read_unlock();
+				if (to_put)
+					iput(to_put);
 				nfs_sb_deactive(server->super);
 				goto restart;
 			}
 			delegation = nfs_start_delegation_return_locked(NFS_I(inode));
 			rcu_read_unlock();
 
+			if (to_put)
+				iput(to_put);
+
 			err = nfs_end_delegation_return(inode, delegation, 0);
 			iput(inode);
 			nfs_sb_deactive(server->super);
@@ -512,10 +557,14 @@ restart:
 			if (!err)
 				goto restart;
 			set_bit(NFS4CLNT_DELEGRETURN, &clp->cl_state);
+			if (place_holder)
+				iput(place_holder);
 			return err;
 		}
 	}
 	rcu_read_unlock();
+	if (place_holder)
+		iput(place_holder);
 	return 0;
 }