SUNRPC: Ensure memory shrinker doesn't waste time in rpcauth_prune_expired()

The 'cred_unused' list, that is traversed by rpcauth_cache_shrinker is
ordered by time. If we hit a credential that is under the 60 second garbage
collection moratorium, we should exit because we know at that point that
all successive credentials are subject to the same moratorium...
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth.c

@@ -236,10 +236,13 @@ rpcauth_prune_expired(struct list_head *free, int nr_to_scan)
 
 	list_for_each_entry_safe(cred, next, &cred_unused, cr_lru) {
 
-		/* Enforce a 60 second garbage collection moratorium */
+		/*
+		 * Enforce a 60 second garbage collection moratorium
+		 * Note that the cred_unused list must be time-ordered.
+		 */
 		if (time_in_range(cred->cr_expire, expired, jiffies) &&
 		    test_bit(RPCAUTH_CRED_HASHED, &cred->cr_flags) != 0)
-			continue;
+			return 0;
 
 		list_del_init(&cred->cr_lru);
 		number_cred_unused--;
@@ -258,7 +261,7 @@ rpcauth_prune_expired(struct list_head *free, int nr_to_scan)
 		if (nr_to_scan == 0)
 			break;
 	}
-	return nr_to_scan;
+	return (number_cred_unused / 100) * sysctl_vfs_cache_pressure;
 }
 
 /*
@@ -275,8 +278,7 @@ rpcauth_cache_shrinker(int nr_to_scan, gfp_t gfp_mask)
 	if (list_empty(&cred_unused))
 		return 0;
 	spin_lock(&rpc_credcache_lock);
-	nr_to_scan = rpcauth_prune_expired(&free, nr_to_scan);
-	res = (number_cred_unused / 100) * sysctl_vfs_cache_pressure;
+	res = rpcauth_prune_expired(&free, nr_to_scan);
 	spin_unlock(&rpc_credcache_lock);
 	rpcauth_destroy_credlist(&free);
 	return res;