From 274132ac23e7f9214bf55065ed7f25e72fa008e5 Mon Sep 17 00:00:00 2001
From: Jeff Roberson <jeff@FreeBSD.org>
Date: Wed, 21 Aug 2013 22:39:19 +0000
Subject: [PATCH]  - Eliminate the vm object lock from the active queue scan. 
 It is not    necessary since we do not free or cache the page from active
 anymore.    Document the one possible race that is harmless.

Sponsored by:	EMC / Isilon Storage Division
Discussed with:	alc
---
 sys/vm/vm_pageout.c | 33 +++++++++------------------------
 1 file changed, 9 insertions(+), 24 deletions(-)

diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c
index 472ae78a31d..5660b561a0f 100644
--- a/sys/vm/vm_pageout.c
+++ b/sys/vm/vm_pageout.c
@@ -1333,25 +1333,6 @@ relock_queues:
 			m = next;
 			continue;
 		}
-		object = m->object;
-		if (!VM_OBJECT_TRYWLOCK(object) &&
-		    !vm_pageout_fallback_object_lock(m, &next)) {
-			VM_OBJECT_WUNLOCK(object);
-			vm_page_unlock(m);
-			m = next;
-			continue;
-		}
-
-		/*
-		 * Don't deactivate pages that are busy.
-		 */
-		if (vm_page_busied(m) || m->hold_count != 0) {
-			vm_page_unlock(m);
-			VM_OBJECT_WUNLOCK(object);
-			vm_page_requeue_locked(m);
-			m = next;
-			continue;
-		}
 
 		/*
 		 * The count for pagedaemon pages is done after checking the
@@ -1367,7 +1348,15 @@ relock_queues:
 			vm_page_aflag_clear(m, PGA_REFERENCED);
 			act_delta += 1;
 		}
-		if (object->ref_count != 0)
+		/*
+		 * Unlocked object ref count check.  Two races are possible.
+		 * 1) The ref was transitioning to zero and we saw non-zero,
+		 *    the pmap bits will be checked unnecessarily.
+		 * 2) The ref was transitioning to one and we saw zero. 
+		 *    The page lock prevents a new reference to this page so
+		 *    we need not check the reference bits.
+		 */
+		if (m->object->ref_count != 0)
 			act_delta += pmap_ts_referenced(m);
 
 		/*
@@ -1387,9 +1376,6 @@ relock_queues:
 		 * queue depending on usage.
 		 */
 		if (act_delta == 0) {
-			KASSERT(object->ref_count != 0 ||
-			    !pmap_page_is_mapped(m),
-			    ("vm_pageout_scan: page %p is mapped", m));
 			/* Dequeue to avoid later lock recursion. */
 			vm_page_dequeue_locked(m);
 			vm_page_deactivate(m);
@@ -1397,7 +1383,6 @@ relock_queues:
 		} else
 			vm_page_requeue_locked(m);
 		vm_page_unlock(m);
-		VM_OBJECT_WUNLOCK(object);
 		m = next;
 	}
 	vm_pagequeue_unlock(pq);