Re: [PATCH 03/18] rhashtable: remove nulls_base and related code.

NeilBrown
2018-06-07T02:49:07Z

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On Fri, Jun 01 2018, NeilBrown wrote:

> This "feature" is unused, undocumented, and untested and so
> doesn't really belong. Next patch will introduce support
> to detect when a search gets diverted down a different chain,
> which the common purpose of nulls markers.
>
> This patch actually fixes a bug too. The table resizing allows a
> table to grow to 2^31 buckets, but the hash is truncated to 27 bits -
> any growth beyond 2^27 is wasteful an ineffective.
>
> This patch results in NULLS_MARKER(0) being used for all chains,
> and leaves the use of rht_is_a_null() to test for it.
>
> Signed-off-by: NeilBrown <neilb@suse.com>

Hi Herbert,
You've acked a few patches that depends on this one, but not this
patch itself. If you could ack this one, I could submit a collection
of patches for inclusion (after the merge window closes I guess)
and then have fewer outstanding.
This assumes you are in-principle happy with the alternative approach I
took to handling list-nulls. I got the impression that it was only
some small details holding that back.

Thanks,
NeilBrown

> ---
> include/linux/rhashtable-types.h | 2 --
> include/linux/rhashtable.h | 33 +++------------------------------
> lib/rhashtable.c | 8 --------
> lib/test_rhashtable.c | 5 +----
> 4 files changed, 4 insertions(+), 44 deletions(-)
>
> diff --git a/include/linux/rhashtable-types.h b/include/linux/rhashtable-types.h
> index 9740063ff13b..763d613ce2c2 100644
> --- a/include/linux/rhashtable-types.h
> +++ b/include/linux/rhashtable-types.h
> @@ -50,7 +50,6 @@ typedef int (*rht_obj_cmpfn_t)(struct rhashtable_compare_arg *arg,
> * @min_size: Minimum size while shrinking
> * @locks_mul: Number of bucket locks to allocate per cpu (default: 32)
> * @automatic_shrinking: Enable automatic shrinking of tables
> - * @nulls_base: Base value to generate nulls marker
> * @hashfn: Hash function (default: jhash2 if !(key_len % 4), or jhash)
> * @obj_hashfn: Function to hash object
> * @obj_cmpfn: Function to compare key with object
> @@ -64,7 +63,6 @@ struct rhashtable_params {
> u16 min_size;
> bool automatic_shrinking;
> u8 locks_mul;
> - u32 nulls_base;
> rht_hashfn_t hashfn;
> rht_obj_hashfn_t obj_hashfn;
> rht_obj_cmpfn_t obj_cmpfn;
> diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
> index 48754ab07cdf..d9f719af7936 100644
> --- a/include/linux/rhashtable.h
> +++ b/include/linux/rhashtable.h
> @@ -28,25 +28,8 @@
> #include <linux/rhashtable-types.h>
> /*
> * The end of the chain is marked with a special nulls marks which has
> - * the following format:
> - *
> - * +-------+-----------------------------------------------------+-+
> - * | Base | Hash |1|
> - * +-------+-----------------------------------------------------+-+
> - *
> - * Base (4 bits) : Reserved to distinguish between multiple tables.
> - * Specified via &struct rhashtable_params.nulls_base.
> - * Hash (27 bits): Full hash (unmasked) of first element added to bucket
> - * 1 (1 bit) : Nulls marker (always set)
> - *
> - * The remaining bits of the next pointer remain unused for now.
> + * the least significant bit set.
> */
> -#define RHT_BASE_BITS 4
> -#define RHT_HASH_BITS 27
> -#define RHT_BASE_SHIFT RHT_HASH_BITS
> -
> -/* Base bits plus 1 bit for nulls marker */
> -#define RHT_HASH_RESERVED_SPACE (RHT_BASE_BITS + 1)
>
> /* Maximum chain length before rehash
> *
> @@ -92,24 +75,14 @@ struct bucket_table {
> struct rhash_head __rcu *buckets[] ____cacheline_aligned_in_smp;
> };
>
> -static inline unsigned long rht_marker(const struct rhashtable *ht, u32 hash)
> -{
> - return NULLS_MARKER(ht->p.nulls_base + hash);
> -}
> -
> #define INIT_RHT_NULLS_HEAD(ptr, ht, hash) \
> - ((ptr) = (typeof(ptr)) rht_marker(ht, hash))
> + ((ptr) = (typeof(ptr)) NULLS_MARKER(0))
>
> static inline bool rht_is_a_nulls(const struct rhash_head *ptr)
> {
> return ((unsigned long) ptr & 1);
> }
>
> -static inline unsigned long rht_get_nulls_value(const struct rhash_head *ptr)
> -{
> - return ((unsigned long) ptr) >> 1;
> -}
> -
> static inline void *rht_obj(const struct rhashtable *ht,
> const struct rhash_head *he)
> {
> @@ -119,7 +92,7 @@ static inline void *rht_obj(const struct rhashtable *ht,
> static inline unsigned int rht_bucket_index(const struct bucket_table *tbl,
> unsigned int hash)
> {
> - return (hash >> RHT_HASH_RESERVED_SPACE) & (tbl->size - 1);
> + return hash & (tbl->size - 1);
> }
>
> static inline unsigned int rht_key_get_hash(struct rhashtable *ht,
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index c9fafea7dc6e..688693c919be 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -995,7 +995,6 @@ static u32 rhashtable_jhash2(const void *key, u32 length, u32 seed)
> * .key_offset = offsetof(struct test_obj, key),
> * .key_len = sizeof(int),
> * .hashfn = jhash,
> - * .nulls_base = (1U << RHT_BASE_SHIFT),
> * };
> *
> * Configuration Example 2: Variable length keys
> @@ -1029,9 +1028,6 @@ int rhashtable_init(struct rhashtable *ht,
> (params->obj_hashfn && !params->obj_cmpfn))
> return -EINVAL;
>
> - if (params->nulls_base && params->nulls_base < (1U << RHT_BASE_SHIFT))
> - return -EINVAL;
> -
> memset(ht, 0, sizeof(*ht));
> mutex_init(&ht->mutex);
> spin_lock_init(&ht->lock);
> @@ -1096,10 +1092,6 @@ int rhltable_init(struct rhltable *hlt, const struct rhashtable_params *params)
> {
> int err;
>
> - /* No rhlist NULLs marking for now. */
> - if (params->nulls_base)
> - return -EINVAL;
> -
> err = rhashtable_init(&hlt->ht, params);
> hlt->ht.rhlist = true;
> return err;
> diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
> index bf92b7aa2a49..b428a9c7522a 100644
> --- a/lib/test_rhashtable.c
> +++ b/lib/test_rhashtable.c
> @@ -83,7 +83,7 @@ static u32 my_hashfn(const void *data, u32 len, u32 seed)
> {
> const struct test_obj_rhl *obj = data;
>
> - return (obj->value.id % 10) << RHT_HASH_RESERVED_SPACE;
> + return (obj->value.id % 10);
> }
>
> static int my_cmpfn(struct rhashtable_compare_arg *arg, const void *obj)
> @@ -99,7 +99,6 @@ static struct rhashtable_params test_rht_params = {
> .key_offset = offsetof(struct test_obj, value),
> .key_len = sizeof(struct test_obj_val),
> .hashfn = jhash,
> - .nulls_base = (3U << RHT_BASE_SHIFT),
> };
>
> static struct rhashtable_params test_rht_params_dup = {
> @@ -294,8 +293,6 @@ static int __init test_rhltable(unsigned int entries)
> if (!obj_in_table)
> goto out_free;
>
> - /* nulls_base not supported in rhlist interface */
> - test_rht_params.nulls_base = 0;
> err = rhltable_init(&rhlt, &test_rht_params);
> if (WARN_ON(err))
> goto out_free;

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