The vmem callback to reclaim kmem arena address space on low or

fragmented conditions currently just wakes up the pagedaemon.  The
kmem arena is significantly smaller then the total available physical
memory, which means that there are loads where kmem arena space could
be exhausted, while there is a lot of pages available still.  The
woken up pagedaemon sees vm_pages_needed != 0, verifies the condition
vm_paging_needed() which is false, clears the pass and returns back to
sleep, not calling neither uma_reclaim() nor lowmem handler.

To handle low kmem arena conditions, create additional pagedaemon
thread which calls uma_reclaim() directly.  The thread sleeps on the
dedicated channel and kmem_reclaim() wakes the thread in addition to
the pagedaemon.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/kern/kern_malloc.c

@@ -665,13 +665,15 @@ reallocf(void *addr, unsigned long size, struct malloc_type *mtp, int flags)
 }
 
 /*
- * Wake the page daemon when we exhaust KVA.  It will call the lowmem handler
- * and uma_reclaim() callbacks in a context that is safe.
+ * Wake the uma reclamation pagedaemon thread when we exhaust KVA.  It
+ * will call the lowmem handler and uma_reclaim() callbacks in a
+ * context that is safe.
  */
 static void
 kmem_reclaim(vmem_t *vm, int flags)
 {
 
+	uma_reclaim_wakeup();
 	pagedaemon_wakeup();
 }
 

sys/vm/uma.h

@@ -690,4 +690,7 @@ struct uma_percpu_stat {
 	uint64_t	_ups_reserved[5];	/* Reserved. */
 };
 
+void uma_reclaim_wakeup(void);
+void uma_reclaim_worker(void *);
+
 #endif	/* _VM_UMA_H_ */

sys/vm/uma_core.c

@@ -3222,16 +3222,17 @@ uma_find_refcnt(uma_zone_t zone, void *item)
 }
 
 /* See uma.h */
-void
-uma_reclaim(void)
+static void
+uma_reclaim_locked(bool kmem_danger)
 {
+
 #ifdef UMA_DEBUG
 	printf("UMA: vm asked us to release pages!\n");
 #endif
-	sx_xlock(&uma_drain_lock);
+	sx_assert(&uma_drain_lock, SA_XLOCKED);
 	bucket_enable();
 	zone_foreach(zone_drain);
-	if (vm_page_count_min()) {
+	if (vm_page_count_min() || kmem_danger) {
 		cache_drain_safe(NULL);
 		zone_foreach(zone_drain);
 	}
@@ -3243,9 +3244,42 @@ uma_reclaim(void)
 	zone_drain(slabzone);
 	zone_drain(slabrefzone);
 	bucket_zone_drain();
+}
+
+void
+uma_reclaim(void)
+{
+
+	sx_xlock(&uma_drain_lock);
+	uma_reclaim_locked(false);
 	sx_xunlock(&uma_drain_lock);
 }
 
+static int uma_reclaim_needed;
+
+void
+uma_reclaim_wakeup(void)
+{
+
+	uma_reclaim_needed = 1;
+	wakeup(&uma_reclaim_needed);
+}
+
+void
+uma_reclaim_worker(void *arg __unused)
+{
+
+	sx_xlock(&uma_drain_lock);
+	for (;;) {
+		sx_sleep(&uma_reclaim_needed, &uma_drain_lock, PVM,
+		    "umarcl", 0);
+		if (uma_reclaim_needed) {
+			uma_reclaim_needed = 0;
+			uma_reclaim_locked(true);
+		}
+	}
+}
+
 /* See uma.h */
 int
 uma_zone_exhausted(uma_zone_t zone)

sys/vm/vm_pageout.c

@@ -1724,8 +1724,9 @@ vm_pageout_init(void)
 static void
 vm_pageout(void)
 {
+	int error;
 #if MAXMEMDOM > 1
-	int error, i;
+	int i;
 #endif
 
 	swap_pager_swap_init();
@@ -1739,6 +1740,10 @@ vm_pageout(void)
 		}
 	}
 #endif
+	error = kthread_add(uma_reclaim_worker, NULL, curproc, NULL,
+	    0, 0, "uma");
+	if (error != 0)
+		panic("starting uma_reclaim helper, error %d\n", error);
 	vm_pageout_worker((void *)(uintptr_t)0);
 }