The synchronization provided by vm object locking has eliminated the

need for most calls to vm_page_busy().  Specifically, most calls to
vm_page_busy() occur immediately prior to a call to vm_page_remove().
In such cases, the containing vm object is locked across both calls.
Consequently, the setting of the vm page's PG_BUSY flag is not even
visible to other threads that are following the synchronization
protocol.

This change (1) eliminates the calls to vm_page_busy() that
immediately precede a call to vm_page_remove() or functions, such as
vm_page_free() and vm_page_rename(), that call it and (2) relaxes the
requirement in vm_page_remove() that the vm page's PG_BUSY flag is
set.  Now, the vm page's PG_BUSY flag is set only when the vm object
lock is released while the vm page is still in transition.  Typically,
this is when it is undergoing I/O.

5 files changed, 5 insertions, 23 deletions

diff --git a/sys/vm/vm_contig.c b/sys/vm/vm_contig.c
index 5ebf1bc..952d26d 100644
--- a/sys/vm/vm_contig.c
+++ b/sys/vm/vm_contig.c
@@ -256,7 +256,6 @@ again1:
 					start++;
 					goto again0;
 				}
-				vm_page_busy(m);
 				vm_page_free(m);
 				VM_OBJECT_UNLOCK(object);
 			}
@@ -459,7 +458,6 @@ cleanup_freed:
 				object = m->object;
 				if (!VM_OBJECT_TRYLOCK(object))
 					goto retry;
-				vm_page_busy(m);
 				vm_page_free(m);
 				VM_OBJECT_UNLOCK(object);
 			}
diff --git a/sys/vm/vm_glue.c b/sys/vm/vm_glue.c
index 47d4c96..1673f2b 100644
--- a/sys/vm/vm_glue.c
+++ b/sys/vm/vm_glue.c
@@ -296,7 +296,6 @@ vm_proc_dispose(struct proc *p)
 		panic("vm_proc_dispose: incorrect number of pages in upobj");
 	vm_page_lock_queues();
 	while ((m = TAILQ_FIRST(&upobj->memq)) != NULL) {
-		vm_page_busy(m);
 		vm_page_unwire(m, 0);
 		vm_page_free(m);
 	}
@@ -496,7 +495,6 @@ vm_thread_dispose(struct thread *td)
 		if (m == NULL)
 			panic("vm_thread_dispose: kstack already missing?");
 		vm_page_lock_queues();
-		vm_page_busy(m);
 		vm_page_unwire(m, 0);
 		vm_page_free(m);
 		vm_page_unlock_queues();
diff --git a/sys/vm/vm_object.c b/sys/vm/vm_object.c
index e697790..ebb3c03 100644
--- a/sys/vm/vm_object.c
+++ b/sys/vm/vm_object.c
@@ -621,11 +621,9 @@ vm_object_terminate(vm_object_t object)
 			("vm_object_terminate: freeing busy page %p "
 			"p->busy = %d, p->flags %x\n", p, p->busy, p->flags));
 		if (p->wire_count == 0) {
-			vm_page_busy(p);
 			vm_page_free(p);
 			cnt.v_pfree++;
 		} else {
-			vm_page_busy(p);
 			vm_page_remove(p);
 		}
 	}
@@ -1803,7 +1801,6 @@ again:
 			if (p->valid & p->dirty)
 				continue;
 		}
-		vm_page_busy(p);
 		pmap_remove_all(p);
 		vm_page_free(p);
 	}
diff --git a/sys/vm/vm_page.c b/sys/vm/vm_page.c
index 5001c78..3b880c9 100644
--- a/sys/vm/vm_page.c
+++ b/sys/vm/vm_page.c
@@ -609,21 +609,15 @@ vm_page_remove(vm_page_t m)
 	vm_page_t root;
 
 	mtx_assert(&vm_page_queue_mtx, MA_OWNED);
-	if (m->object == NULL)
+	if ((object = m->object) == NULL)
 		return;
-	VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED);
-	if ((m->flags & PG_BUSY) == 0) {
-		panic("vm_page_remove: page not busy");
+	VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
+	if (m->flags & PG_BUSY) {
+		vm_page_flag_clear(m, PG_BUSY);
+		vm_page_flash(m);
 	}
 
 	/*
-	 * Basically destroy the page.
-	 */
-	vm_page_wakeup(m);
-
-	object = m->object;
-
-	/*
 	 * Now remove from the object's list of backed pages.
 	 */
 	if (m != object->root)
@@ -810,7 +804,6 @@ loop:
 		}
 		m_object = m->object;
 		VM_OBJECT_LOCK_ASSERT(m_object, MA_OWNED);
-		vm_page_busy(m);
 		vm_page_free(m);
 		vm_page_unlock_queues();
 		if (m_object != object)
@@ -1298,7 +1291,6 @@ vm_page_try_to_free(vm_page_t m)
 	pmap_remove_all(m);
 	if (m->dirty)
 		return (0);
-	vm_page_busy(m);
 	vm_page_free(m);
 	return (1);
 }
@@ -1667,7 +1659,6 @@ vm_page_cowfault(vm_page_t m)
 	pindex = m->pindex;
 
  retry_alloc:
-	vm_page_busy(m);
 	vm_page_remove(m);
 	mnew = vm_page_alloc(object, pindex, VM_ALLOC_NORMAL);
 	if (mnew == NULL) {
diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c
index 3d21c1f..cfb9eab 100644
--- a/sys/vm/vm_pageout.c
+++ b/sys/vm/vm_pageout.c
@@ -823,7 +823,6 @@ rescan0:
 			/*
 			 * Invalid pages can be easily freed
 			 */
-			vm_page_busy(m);
 			pmap_remove_all(m);
 			vm_page_free(m);
 			cnt.v_dfree++;
@@ -1107,7 +1106,6 @@ unlock_and_continue:
 		cache_rover = (m->pc + PQ_PRIME2) & PQ_L2_MASK;
 		object = m->object;
 		VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
-		vm_page_busy(m);
 		vm_page_free(m);
 		VM_OBJECT_UNLOCK(object);
 		cnt.v_dfree++;