In the common case, pmap_enter_quick() completes without sleeping.

In such cases, the busying of the page and the unlocking of the
containing object by vm_map_pmap_enter() and vm_fault_prefault() is
unnecessary overhead.  To eliminate this overhead, this change
modifies pmap_enter_quick() so that it expects the object to be locked
on entry and it assumes the responsibility for busying the page and
unlocking the object if it must sleep.  Note: alpha, amd64, i386 and
ia64 are the only implementations optimized by this change; arm,
powerpc, and sparc64 still conservatively busy the page and unlock the
object within every pmap_enter_quick() call.

Additionally, this change is the first case where we synchronize
access to the page's PG_BUSY flag and busy field using the containing
object's lock rather than the global page queues lock.  (Modifications
to the page's PG_BUSY flag and busy field have asserted both locks for
several weeks, enabling an incremental transition.)

1 files changed, 12 insertions, 2 deletions

diff --git a/sys/i386/i386/pmap.c b/sys/i386/i386/pmap.c
index d16bf02..67ac6f6 100644
--- a/sys/i386/i386/pmap.c
+++ b/sys/i386/i386/pmap.c
@@ -2094,9 +2094,19 @@ retry:
 				mpte->wire_count++;
 			} else {
 				mpte = _pmap_allocpte(pmap, ptepindex,
-				    M_WAITOK);
-				if (mpte == NULL)
+				    M_NOWAIT);
+				if (mpte == NULL) {
+					PMAP_UNLOCK(pmap);
+					vm_page_busy(m);
+					vm_page_unlock_queues();
+					VM_OBJECT_UNLOCK(m->object);
+					VM_WAIT;
+					VM_OBJECT_LOCK(m->object);
+					vm_page_lock_queues();
+					vm_page_wakeup(m);
+					PMAP_LOCK(pmap);
 					goto retry;
+				}
 			}
 		}
 	} else {