SUNRPC: Enforce an upper limit on the number of cached credentials

In some cases where the credentials are not often reused, we may want
to limit their total number just in order to make the negative lookups
in the hash table more manageable.
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/auth.c

@@ -80,6 +80,10 @@ static struct kernel_param_ops param_ops_hashtbl_sz = {
 module_param_named(auth_hashtable_size, auth_hashbits, hashtbl_sz, 0644);
 MODULE_PARM_DESC(auth_hashtable_size, "RPC credential cache hashtable size");
 
+static unsigned long auth_max_cred_cachesize = ULONG_MAX;
+module_param(auth_max_cred_cachesize, ulong, 0644);
+MODULE_PARM_DESC(auth_max_cred_cachesize, "RPC credential maximum total cache size");
+
 static u32
 pseudoflavor_to_flavor(u32 flavor) {
 	if (flavor > RPC_AUTH_MAXFLAVOR)
@@ -481,6 +485,20 @@ rpcauth_prune_expired(struct list_head *free, int nr_to_scan)
 	return freed;
 }
 
+static unsigned long
+rpcauth_cache_do_shrink(int nr_to_scan)
+{
+	LIST_HEAD(free);
+	unsigned long freed;
+
+	spin_lock(&rpc_credcache_lock);
+	freed = rpcauth_prune_expired(&free, nr_to_scan);
+	spin_unlock(&rpc_credcache_lock);
+	rpcauth_destroy_credlist(&free);
+
+	return freed;
+}
+
 /*
  * Run memory cache shrinker.
  */
@@ -488,9 +506,6 @@ static unsigned long
 rpcauth_cache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 
 {
-	LIST_HEAD(free);
-	unsigned long freed;
-
 	if ((sc->gfp_mask & GFP_KERNEL) != GFP_KERNEL)
 		return SHRINK_STOP;
 
@@ -498,12 +513,7 @@ rpcauth_cache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 	if (list_empty(&cred_unused))
 		return SHRINK_STOP;
 
-	spin_lock(&rpc_credcache_lock);
-	freed = rpcauth_prune_expired(&free, sc->nr_to_scan);
-	spin_unlock(&rpc_credcache_lock);
-	rpcauth_destroy_credlist(&free);
-
-	return freed;
+	return rpcauth_cache_do_shrink(sc->nr_to_scan);
 }
 
 static unsigned long
@@ -513,6 +523,21 @@ rpcauth_cache_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 	return (number_cred_unused / 100) * sysctl_vfs_cache_pressure;
 }
 
+static void
+rpcauth_cache_enforce_limit(void)
+{
+	unsigned long diff;
+	unsigned int nr_to_scan;
+
+	if (number_cred_unused <= auth_max_cred_cachesize)
+		return;
+	diff = number_cred_unused - auth_max_cred_cachesize;
+	nr_to_scan = 100;
+	if (diff < nr_to_scan)
+		nr_to_scan = diff;
+	rpcauth_cache_do_shrink(nr_to_scan);
+}
+
 /*
  * Look up a process' credentials in the authentication cache
  */
@@ -566,6 +591,7 @@ rpcauth_lookup_credcache(struct rpc_auth *auth, struct auth_cred * acred,
 	} else
 		list_add_tail(&new->cr_lru, &free);
 	spin_unlock(&cache->lock);
+	rpcauth_cache_enforce_limit();
 found:
 	if (test_bit(RPCAUTH_CRED_NEW, &cred->cr_flags) &&
 	    cred->cr_ops->cr_init != NULL &&