Index: vm/vm_fault.c
===================================================================
--- vm/vm_fault.c
+++ vm/vm_fault.c
@@ -132,7 +132,7 @@
 static void vm_fault_dontneed(const struct faultstate *fs, vm_offset_t vaddr,
 	    int ahead);
 static void vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
-	    int backward, int forward, bool obj_locked);
+	    int backward, int forward, bool obj_locked, int faultcount);
 
 static inline void
 release_page(struct faultstate *fs)
@@ -323,7 +323,7 @@
 	vm_fault_fill_hold(m_hold, m);
 	vm_fault_dirty(fs->entry, m, prot, fault_type, fault_flags, false);
 	if (psind == 0 && !wired)
-		vm_fault_prefault(fs, vaddr, PFBAK, PFFOR, true);
+		vm_fault_prefault(fs, vaddr, PFBAK, PFFOR, true, 0);
 	VM_OBJECT_RUNLOCK(fs->first_object);
 	vm_map_lookup_done(fs->map, fs->entry);
 	curthread->td_ru.ru_minflt++;
@@ -1181,6 +1181,9 @@
 			 */
 			vm_object_pip_wakeup(fs.object);
 			VM_OBJECT_WUNLOCK(fs.object);
+			if (faultcount == 0)
+				faultcount = -1;
+
 			/*
 			 * Only use the new page below...
 			 */
@@ -1289,7 +1292,7 @@
 	    wired == 0)
 		vm_fault_prefault(&fs, vaddr,
 		    faultcount > 0 ? behind : PFBAK,
-		    faultcount > 0 ? ahead : PFFOR, false);
+		    faultcount > 0 ? ahead : PFFOR, false, faultcount);
 	VM_OBJECT_WLOCK(fs.object);
 	vm_page_lock(fs.m);
 
@@ -1413,6 +1416,17 @@
 		VM_OBJECT_WUNLOCK(first_object);
 }
 
+static SYSCTL_NODE(_debug, OID_AUTO, counters, CTLFLAG_RD, 0, "");
+
+static long counter_negati;
+SYSCTL_ULONG(_debug_counters, OID_AUTO, negati, CTLFLAG_RD, &counter_negati, 0, "");
+
+static long counter_nonneg;
+SYSCTL_ULONG(_debug_counters, OID_AUTO, nonneg, CTLFLAG_RD, &counter_nonneg, 0, "");
+
+static long counter_negmap;
+SYSCTL_ULONG(_debug_counters, OID_AUTO, negmap, CTLFLAG_RD, &counter_negmap, 0, "");
+
 /*
  * vm_fault_prefault provides a quick way of clustering
  * pagefaults into a processes address space.  It is a "cousin"
@@ -1421,7 +1435,7 @@
  */
 static void
 vm_fault_prefault(const struct faultstate *fs, vm_offset_t addra,
-    int backward, int forward, bool obj_locked)
+    int backward, int forward, bool obj_locked, int faultcount)
 {
 	pmap_t pmap;
 	vm_map_entry_t entry;
@@ -1462,6 +1476,10 @@
 		if (addr < starta || addr >= entry->end)
 			continue;
 
+		if (faultcount == -1)
+			atomic_add_long(&counter_negati, 1);
+		else
+			atomic_add_long(&counter_nonneg, 1);
 		if (!pmap_is_prefaultable(pmap, addr))
 			continue;
 
@@ -1486,8 +1504,11 @@
 			break;
 		}
 		if (m->valid == VM_PAGE_BITS_ALL &&
-		    (m->flags & PG_FICTITIOUS) == 0)
+		    (m->flags & PG_FICTITIOUS) == 0) {
 			pmap_enter_quick(pmap, addr, m, entry->protection);
+			if (faultcount == -1)
+				atomic_add_long(&counter_negmap, 1);
+		}
 		if (!obj_locked || lobject != entry->object.vm_object)
 			VM_OBJECT_RUNLOCK(lobject);
 	}