rhashtable: Introduce max_size/min_size

This patch adds the parameters max_size and min_size which are
meant to replace max_shift and min_shift.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rhashtable.h

@@ -85,6 +85,8 @@ struct rhashtable;
  * @head_offset: Offset of rhash_head in struct to be hashed
  * @max_shift: Maximum number of shifts while expanding
  * @min_shift: Minimum number of shifts while shrinking
+ * @max_size: Maximum size while expanding
+ * @min_size: Minimum size while shrinking
  * @nulls_base: Base value to generate nulls marker
  * @locks_mul: Number of bucket locks to allocate per cpu (default: 128)
  * @hashfn: Function to hash key
@@ -97,6 +99,8 @@ struct rhashtable_params {
 	size_t			head_offset;
 	size_t			max_shift;
 	size_t			min_shift;
+	unsigned int		max_size;
+	unsigned int		min_size;
 	u32			nulls_base;
 	size_t			locks_mul;
 	rht_hashfn_t		hashfn;

lib/rhashtable.c

@@ -27,7 +27,7 @@
 #include <linux/err.h>
 
 #define HASH_DEFAULT_SIZE	64UL
-#define HASH_MIN_SIZE		4UL
+#define HASH_MIN_SIZE		4U
 #define BUCKET_LOCKS_PER_CPU   128UL
 
 /* Base bits plus 1 bit for nulls marker */
@@ -188,7 +188,8 @@ static bool rht_grow_above_75(const struct rhashtable *ht,
 {
 	/* Expand table when exceeding 75% load */
 	return atomic_read(&ht->nelems) > (tbl->size / 4 * 3) &&
-	       (!ht->p.max_shift || tbl->size < (1 << ht->p.max_shift));
+	       (!ht->p.max_shift || tbl->size < (1 << ht->p.max_shift)) &&
+	       (!ht->p.max_size || tbl->size < ht->p.max_size);
 }
 
 /**
@@ -201,7 +202,8 @@ static bool rht_shrink_below_30(const struct rhashtable *ht,
 {
 	/* Shrink table beneath 30% load */
 	return atomic_read(&ht->nelems) < (tbl->size * 3 / 10) &&
-	       tbl->size > (1 << ht->p.min_shift);
+	       tbl->size > (1 << ht->p.min_shift) &&
+	       tbl->size > ht->p.min_size;
 }
 
 static int rhashtable_rehash_one(struct rhashtable *ht, unsigned old_hash)
@@ -873,7 +875,8 @@ EXPORT_SYMBOL_GPL(rhashtable_walk_stop);
 static size_t rounded_hashtable_size(struct rhashtable_params *params)
 {
 	return max(roundup_pow_of_two(params->nelem_hint * 4 / 3),
-		   1UL << params->min_shift);
+		   max(1UL << params->min_shift,
+		       (unsigned long)params->min_size));
 }
 
 /**
@@ -935,6 +938,7 @@ int rhashtable_init(struct rhashtable *ht, struct rhashtable_params *params)
 
 	params->min_shift = max_t(size_t, params->min_shift,
 				  ilog2(HASH_MIN_SIZE));
+	params->min_size = max(params->min_size, HASH_MIN_SIZE);
 
 	if (params->nelem_hint)
 		size = rounded_hashtable_size(params);