[Question]: reqsk table size limited to 16?

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

Can someone please have a look, it may be that I am missing something?

It seems that in the following the maximum number of table entries is set
to always 16, despite sysctl_max_syn_backlog (tcp_max_syn_backlog), 
overriding the 'backlog' parameter to listen(2).

net/core/request_sock.c
-----------------------

int reqsk_queue_alloc(struct request_sock_queue *queue,
                      unsigned int nr_table_entries)
{
        size_t lopt_size = sizeof(struct listen_sock);
        struct listen_sock *lopt;

	nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);
        nr_table_entries = max_t(u32, nr_table_entries, 8);
        nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);

	//...
	for (lopt->max_qlen_log = 3;
             (1 << lopt->max_qlen_log) < nr_table_entries;
             lopt->max_qlen_log++);

 	//...
	lopt->nr_table_entries = nr_table_entries;
	
	//...
	return 0
}

The function is called with an argument 'nr_table_entries', which is then clamped as

   sysctl_max_syn_backlog <= nr_table_entries <= 8

If nr_table_entries = 8, then round_pow_of_two(8 + 1) = 16.

The sysctl value is set to a much higher value (default 128 or 1024, net/ipv4/tcp.c).

The reqsk_queue_alloc() gets 'nr_table_entries' passed directly from inet_csk_listen_start(),
which in turn gets its 'nr_table_entries' as the 'backlog' argument to listen(2) via
 * net/dccp/proto.c   (dccp_listen_start) or
 * net/ipv4/af_inet.c (inet_listen).