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Date:      Wed, 21 Aug 2013 22:39:20 +0000 (UTC)
From:      Jeff Roberson <jeff@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r254622 - head/sys/vm
Message-ID:  <201308212239.r7LMdK96043768@svn.freebsd.org>
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Author: jeff
Date: Wed Aug 21 22:39:19 2013
New Revision: 254622
URL: http://svnweb.freebsd.org/changeset/base/254622

Log:
   - 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

Modified:
  head/sys/vm/vm_pageout.c

Modified: head/sys/vm/vm_pageout.c
==============================================================================
--- head/sys/vm/vm_pageout.c	Wed Aug 21 22:37:15 2013	(r254621)
+++ head/sys/vm/vm_pageout.c	Wed Aug 21 22:39:19 2013	(r254622)
@@ -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);

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