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.

1 files changed, 5 insertions, 14 deletions

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) {