rhashtable: Add comment on choice of elasticity value

This patch adds a comment on the choice of the value 16 as the
maximum chain length before we force a rehash.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

lib/rhashtable.c

@@ -736,6 +736,18 @@ int rhashtable_init(struct rhashtable *ht,
 
 	ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE);
 
+	/* The maximum (not average) chain length grows with the
+	 * size of the hash table, at a rate of (log N)/(log log N).
+	 * The value of 16 is selected so that even if the hash
+	 * table grew to 2^32 you would not expect the maximum
+	 * chain length to exceed it unless we are under attack
+	 * (or extremely unlucky).
+	 *
+	 * As this limit is only to detect attacks, we don't need
+	 * to set it to a lower value as you'd need the chain
+	 * length to vastly exceed 16 to have any real effect
+	 * on the system.
+	 */
 	if (!params->insecure_elasticity)
 		ht->elasticity = 16;