vm_swapout: Restore handling of RLIMIT_RSS

In commit 13a1129d70, I removed the mechanism by which the pagedaemon
signals the swapout thread when page reclamation is unable to keep up
with demand.  This is because the swapout thread's main action in this
case is to swap out sleeping processes, but we removed this support.

However, it had the secondary effect of causing the swapout thread to
enforce RLIMIT_RSS when racct is not enabled.  Without it, if
racct_enabled is false, nothing ever kicks the swapout thread.

Restore the old behaviour of trying to enforce RLIMIT_RSS when the page
daemon is unable to keep up with demand.  I'm not at all convinced this
is a good way to implement the limit, but the change wasn't intentional,
so let's restore it for now.

Fixes:		13a1129d70 ("vm: Remove kernel stack swapping support, part 1")
Reviewed by:	olce, kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D56142

sys/conf/files

@@ -5413,7 +5413,7 @@ vm/vm_pager.c			standard
 vm/vm_phys.c			standard
 vm/vm_radix.c			standard
 vm/vm_reserv.c			standard
-vm/vm_swapout.c			optional racct
+vm/vm_swapout.c			standard
 vm/vm_unix.c			standard
 vm/vnode_pager.c		standard
 xen/features.c			optional xenhvm

sys/vm/vm_pageout.c

@@ -1683,7 +1683,7 @@ reinsert:
  * results to present a coherent view of paging activity on this domain.
  */
 static int
-vm_pageout_inactive_dispatch(struct vm_domain *vmd, int shortage)
+vm_pageout_inactive_dispatch(struct vm_domain *vmd, u_int shortage)
 {
 	u_int freed, pps, slop, threads, us;
 
@@ -1796,6 +1796,12 @@ vm_pageout_inactive(struct vm_domain *vmd, int shortage, int *addl_shortage)
 	 */
 	vm_pageout_mightbe_oom(vmd, page_shortage, starting_page_shortage);
 
+	/*
+	 * Kick the swapout thread to enforce RSS limits.
+	 */
+	if (page_shortage > 0)
+		vm_swapout_run();
+
 	/*
 	 * See the description of addl_page_shortage above.
 	 */

sys/vm/vm_pageout.h

@@ -103,5 +103,7 @@ void vm_wait_severe(void);
 int vm_pageout_flush(vm_page_t *mc, int count, int flags, bool *eio);
 void vm_pageout_oom(int shortage);
 
+void vm_swapout_run(void);
+
 #endif /* _KERNEL */
 #endif	/* _VM_VM_PAGEOUT_H_ */

sys/vm/vm_swapout.c

@@ -135,6 +135,19 @@ MTX_SYSINIT(vm_daemon, &vm_daemon_mtx, "vm daemon", MTX_DEF);
 static void vm_swapout_map_deactivate_pages(vm_map_t, long);
 static void vm_swapout_object_deactivate(pmap_t, vm_object_t, long);
 
+void
+vm_swapout_run(void)
+{
+	static int lastrun = 0;
+
+	mtx_lock(&vm_daemon_mtx);
+	if (ticks - lastrun >= hz || ticks < lastrun) {
+		wakeup(&vm_daemon_needed);
+		lastrun = ticks;
+	}
+	mtx_unlock(&vm_daemon_mtx);
+}
+
 static void
 vm_swapout_object_deactivate_page(pmap_t pmap, vm_page_t m, bool unmap)
 {
@@ -287,15 +300,14 @@ vm_daemon(void)
 	if (racct_enable && vm_daemon_timeout == 0)
 		vm_daemon_timeout = hz;
 
-	while (TRUE) {
+	while (true) {
 		mtx_lock(&vm_daemon_mtx);
 		msleep(&vm_daemon_needed, &vm_daemon_mtx, PPAUSE, "psleep",
 		    vm_daemon_timeout);
 		mtx_unlock(&vm_daemon_mtx);
 
 		/*
-		 * scan the processes for exceeding their rlimits or if
-		 * process is swapped out -- deactivate pages
+		 * Scan for processes exceeding their rlimits.
 		 */
 		tryagain = 0;
 		attempts = 0;