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rhashtable: Cap total number of entries to 2^31

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When max_size is not set or if it set to a sufficiently large
value, the nelems counter can overflow.  This would cause havoc
with the automatic shrinking as it would then attempt to fit a
huge number of entries into a tiny hash table.

This patch fixes this by adding max_elems to struct rhashtable
to cap the number of elements.  This is set to 2^31 as nelems is
not a precise count.  This is sufficiently smaller than UINT_MAX
that it should be safe.

When max_size is set max_elems will be lowered to at most twice
max_size as is the status quo.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Herbert Xu 2017-04-27 13:44:51 +08:00 committed by David S. Miller
parent 4b726e81da
commit 6d684e5469
2 changed files with 8 additions and 2 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

5
include/linux/rhashtable.h
View file

@ -155,6 +155,7 @@ struct rhashtable_params {
* @nelems: Number of elements in table * @nelems: Number of elements in table
* @key_len: Key length for hashfn * @key_len: Key length for hashfn
* @p: Configuration parameters * @p: Configuration parameters
* @max_elems: Maximum number of elements in table
* @rhlist: True if this is an rhltable * @rhlist: True if this is an rhltable
* @run_work: Deferred worker to expand/shrink asynchronously * @run_work: Deferred worker to expand/shrink asynchronously
* @mutex: Mutex to protect current/future table swapping * @mutex: Mutex to protect current/future table swapping
@ -165,6 +166,7 @@ struct rhashtable {
atomic_t nelems; atomic_t nelems;
unsigned int key_len; unsigned int key_len;
struct rhashtable_params p; struct rhashtable_params p;
unsigned int max_elems;
bool rhlist; bool rhlist;
struct work_struct run_work; struct work_struct run_work;
struct mutex mutex; struct mutex mutex;
@ -327,8 +329,7 @@ static inline bool rht_grow_above_100(const struct rhashtable *ht,
static inline bool rht_grow_above_max(const struct rhashtable *ht, static inline bool rht_grow_above_max(const struct rhashtable *ht,
const struct bucket_table *tbl) const struct bucket_table *tbl)
{ {
return ht->p.max_size && return atomic_read(&ht->nelems) >= ht->max_elems;
(atomic_read(&ht->nelems) / 2u) >= ht->p.max_size;
} }
/* The bucket lock is selected based on the hash and protects mutations /* The bucket lock is selected based on the hash and protects mutations

5
lib/rhashtable.c
View file

@ -961,6 +961,11 @@ int rhashtable_init(struct rhashtable *ht,
if (params->max_size) if (params->max_size)
ht->p.max_size = rounddown_pow_of_two(params->max_size); ht->p.max_size = rounddown_pow_of_two(params->max_size);
/* Cap total entries at 2^31 to avoid nelems overflow. */
ht->max_elems = 1u << 31;
if (ht->p.max_size < ht->max_elems / 2)
ht->max_elems = ht->p.max_size * 2;
ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE); ht->p.min_size = max(ht->p.min_size, HASH_MIN_SIZE);
if (params->nelem_hint) if (params->nelem_hint)