TIME_WAIT sockets clog system (part 2)

[Steven M. SchultzY]

Subject: TIME_WAIT sockets clog system (part 2)
Index:	sys/sys/uipc_mbuf.c 2.10BSD

Description:
	Sockets in a TIME_WAIT state can constipate the networking
	buffer memory when generated in rapid succession by, for
	example, an "mget" in an ftp session.  If more than a dozen
	or so small files are transferred in rapid succession over
	an ethernet, all the mbufs in the system will be taken up
	by sockets in a TIME_WAIT state (from the socket opened for
	each data transfer).

Repeat-By:
	ftp in to a 2.10.1BSD system, do an "mget *" in a largish
	directory.  note that the transfer will hang/develope problems
	after about a dozen to twenty files.  the 2.10.1BSD system
	has run out of mbufs and will recover in a minute or so (hopefully).
	It should be noted that even a Vax could be run out of mbufs
	if the directory were large enough and network memory was full
	due to other causes.

Fix:
	Apply this patch to /sys/sys/uipc_mbuf.c.  The effect of this fix
	is to call the drain routines at least once - the earlier version
	of m_more() checked the waitability state and rejected the 
	request immediately rather than attempt non-blocking methods
	of freeing up memory first.

	It is (as far as i can see - and yes, these fixes have been tested)
	safe to call the drain routines at this point for two reasons:  
	1) the tcp_drain() routine that existed prior to part 1 of this 
	fix did nothing AND the NEW tcp_drain() routine is guaranteed not 
	to sleep(), 2) the ip_drain() routine only releases fragments and 
	does not sleep() at any point along the way.

	With both part 1 and 2 installed a "ftp mget *" of /etc was done
	with no problems and 8 calls to the drain routines (as reported
	by 'netstat -s').

*** uipc_mbuf.old	Tue Jun 27 23:20:46 1989
--- uipc_mbuf.c	Tue Jun 27 23:23:14 1989
***************
*** 164,177 ****
  {
  	register struct mbuf *m;
  
- 	if (canwait == M_DONTWAIT) {
- 		mbstat.m_drops++;
- 		return (NULL);
- 	}
  	for(;;) {
  		m_expand(canwait);
  		if (mfree)
  			break;
  		mbstat.m_wait++;
  		m_want++;
  		SLEEP((caddr_t)&mfree, PZERO - 1);
--- 164,177 ----
  {
  	register struct mbuf *m;
  
  	for(;;) {
  		m_expand(canwait);
  		if (mfree)
  			break;
+ 		if (canwait == M_DONTWAIT) {
+ 			mbstat.m_drops++;
+ 			return (NULL);
+ 		}
  		mbstat.m_wait++;
  		m_want++;
  		SLEEP((caddr_t)&mfree, PZERO - 1);