RE: Problem with rte_hash_hash when sharing the list between processes

["Pathak, Pravin" <pravin.pathak@intel.com>]

Please refer to https://doc.dpdk.org/guides-1.8/prog_guide/multi_proc_support.html
You may have to disable ASLR to get the same address in the secondary process.

There are a number of limitations to what can be done when running DPDK multi-process applications. Some of these are documented below:
The multi-process feature requires that the exact same hugepage memory mappings be present in all applications. The Linux security feature - Address-Space Layout Randomization (ASLR) can interfere with this mapping, so it may be necessary to disable this feature in order to reliably run multi-process applications.

> -----Original Message-----
> From: Felipe Pereira <felipe@spint.com.br>
> Sent: Saturday, February 15, 2025 9:06 AM
> To: users@dpdk.org
> Subject: Problem with rte_hash_hash when sharing the list between processes
> 
> Hello,
> 
> It's my first email on mailing lists, if I did something wrong please tell me so I will
> not do it again.
> 
> I'm having a problem when using an hash_list shared between processes.
> 
> I have the following code to create the hash_list on the primary process, and get
> the existing list from de secondary process:
> 
> static void
> init_connections_hash(struct rte_hash **connections_hash, int
> max_connections)
> {
>      struct rte_hash_parameters hash_params = {
>          .name = HASH_NAME,
>          .entries = max_connections,
>          .key_len = sizeof(struct connection),
>          .hash_func = rte_hash_crc,
>          .hash_func_init_val = 0,
>          .socket_id = SOCKET_ID_ANY,
>          .extra_flag =
> RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD|RTE_HASH_EXTRA_FLAGS_RW
> _CONCURRENCY_LF
>      };
>      *connections_hash = rte_hash_create(&hash_params);
>      if (*connections_hash == NULL)
>      {
>          rte_exit(EXIT_FAILURE, "Failed to create TCP connections hash table\n");
>      }
> 
> }
> 
> static void
> get_existing_connections_hash(struct rte_hash **connections_hash) {
>      *connections_hash = rte_hash_find_existing(HASH_NAME);
>      if (*connections_hash == NULL)
>      {
>          rte_exit(EXIT_FAILURE, "Failed to get existing TCP connections hash
> table\n");
>      }
> }
> 
> And in the main, when initializing the process:
> 
> if(proc_type == RTE_PROC_PRIMARY) {
>          init_connections_hash(&connections_hash, MAX_CONNECTIONS_IN_LIST);
>      } else {
>          get_existing_connections_hash(&connections_hash);
>      }
> }
> 
> What I think it's happening, is when I create with .hash_func=rte_hash_crc (I also
> tested with rte_jhash), it writes the memory address of the function, that exists or
> is allocated on the main process, but is not shared with the secondary process.
> 
> So when I call rte_hash_lookup_data, it calls rte_hash_hash(h, key), and for
> reference the function is:
> 
> hash_sig_t
> rte_hash_hash(const struct rte_hash *h, const void *key) {
>      /* calc hash result by key */
>      return h->hash_func(key, h->key_len, h->hash_func_init_val); }
> 
> But h->hash_func is the memory address that is available only in the primary
> process, and when it gets called at the secondary process, I get segmentation
> fault error.
> 
> I tried changing the h->hash_func to rte_hash_crc, for my application it worked
> like a charm, so I really think it's something with the address of the function not
> being available to the secondary process.
> 
> This only happens with this function, I tried to add some items in the primary
> process, and in the secondary I iterated the list with rte_hash_iterate, I could read
> everything in it without any problem, so the has_list was shared correctly
> between the processes.
> 
> The solution I found was to use rte_hash_lookup_with_hash_data instead of
> rte_hash_lookup_data, it's working like I intended, but then it would be pointless
> to init the hash with .hash_func=somehashfunc when using multiple processes
> 
> So I would like to know if it's something wrong that I did when creating or using
> the list, if it is some bug on the code, or if this solution is really the way it's
> intended to do when using multiple processes.
> 
> Thanks