erts: Limit the automatic max buffer for UDP to 2^16

There is no reason to have a larger buffer than this as
the recvmsg call will never return more data.

OTP-15206

2 files changed, 20 insertions, 11 deletions

diff --git a/lib/kernel/doc/src/inet.xml b/lib/kernel/doc/src/inet.xml
index e6a7962c5a..f281d61459 100644
--- a/lib/kernel/doc/src/inet.xml
+++ b/lib/kernel/doc/src/inet.xml
@@ -734,22 +734,23 @@ get_tcpi_sacked(Sock) ->
           </item>
           <tag><c>{buffer, Size}</c></tag>
           <item>
-            <p>The size of the user-level software buffer used by
-              the driver. 
-              Not to be confused with options <c>sndbuf</c>
+            <p>The size of the user-level buffer used by
+              the driver. Not to be confused with options <c>sndbuf</c>
               and <c>recbuf</c>, which correspond to the
-              Kernel socket buffers. It is recommended
-              to have <c>val(buffer) &gt;= max(val(sndbuf),val(recbuf))</c> to
+              Kernel socket buffers. For TCP it is recommended
+              to have <c>val(buffer) &gt;= val(recbuf)</c> to
               avoid performance issues because of unnecessary copying.
+              For UDP the same recommendation applies, but the max should
+              not be larger than the MTU of the network path.
               <c>val(buffer)</c> is automatically set to the above
-              maximum when values <c>sndbuf</c> or <c>recbuf</c> are set.
-              However, as the sizes set for <c>sndbuf</c> and <c>recbuf</c>
+              maximum when <c>recbuf</c> is set.
+              However, as the size set for <c>recbuf</c>
               usually become larger, you are encouraged to use
               <seealso marker="#getopts/2"><c>getopts/2</c></seealso>
               to analyze the behavior of your operating system.</p>
             <p>Note that this is also the maximum amount of data that can be
-	       received from a single recv call. If you are using higher than 
-	       normal MTU consider setting buffer higher.</p> 
+	       received from a single recv call. If you are using higher than
+	       normal MTU consider setting buffer higher.</p>
           </item>
           <tag><c>{delay_send, Boolean}</c></tag>
           <item>
diff --git a/lib/kernel/test/gen_udp_SUITE.erl b/lib/kernel/test/gen_udp_SUITE.erl
index b39399b18a..3acfff929e 100644
--- a/lib/kernel/test/gen_udp_SUITE.erl
+++ b/lib/kernel/test/gen_udp_SUITE.erl
@@ -34,7 +34,7 @@
 -export([init_per_testcase/2, end_per_testcase/2]).
 
 -export([send_to_closed/1, active_n/1,
-	 buffer_size/1, binary_passive_recv/1, bad_address/1,
+	 buffer_size/1, binary_passive_recv/1, max_buffer_size/1, bad_address/1,
 	 read_packets/1, open_fd/1, connect/1, implicit_inet6/1,
 	 local_basic/1, local_unbound/1,
 	 local_fdopen/1, local_fdopen_unbound/1, local_abstract/1]).
@@ -44,7 +44,7 @@ suite() ->
      {timetrap,{minutes,1}}].
 
 all() -> 
-    [send_to_closed, buffer_size, binary_passive_recv,
+    [send_to_closed, buffer_size, binary_passive_recv, max_buffer_size,
      bad_address, read_packets, open_fd, connect,
      implicit_inet6, active_n,
      {group, local}].
@@ -237,6 +237,14 @@ buffer_size_server_recv(Socket, IP, Port, Cnt) ->
     end.
 
 
+%%-------------------------------------------------------------
+%% OTP-15206: Keep buffer small for udp
+%%-------------------------------------------------------------
+max_buffer_size(Config) when is_list(Config) ->
+    {ok, Socket}  = gen_udp:open(0, [binary]),
+    ok = inet:setopts(Socket,[{recbuf, 1 bsl 20}]),
+    {ok, [{buffer, 65536}]} = inet:getopts(Socket,[buffer]),
+    gen_udp:close(Socket).
 
 %%-------------------------------------------------------------
 %% OTP-3823 gen_udp:recv does not return address in binary mode