o Relax locking assertions for vm_page_find_least()

o Relax locking assertions for pmap_enter_object() and add them also
  to architectures that currently don't have any
o Introduce VM_OBJECT_LOCK_DOWNGRADE() which is basically a downgrade
  operation on the per-object rwlock
o Use all the mechanisms above to make vm_map_pmap_enter() to work
  mostl of the times only with readlocks.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	alc

3 files changed, 18 insertions, 8 deletions

diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c
index 933b0e1..a9ae803 100644
--- a/sys/vm/vm_map.c
+++ b/sys/vm/vm_map.c
@@ -1806,18 +1806,27 @@ vm_map_pmap_enter(vm_map_t map, vm_offset_t addr, vm_prot_t prot,
 
 	if ((prot & (VM_PROT_READ | VM_PROT_EXECUTE)) == 0 || object == NULL)
 		return;
-	VM_OBJECT_WLOCK(object);
+	VM_OBJECT_RLOCK(object);
 	if (object->type == OBJT_DEVICE || object->type == OBJT_SG) {
-		pmap_object_init_pt(map->pmap, addr, object, pindex, size);
-		goto unlock_return;
+		VM_OBJECT_RUNLOCK(object);
+		VM_OBJECT_WLOCK(object);
+		if (object->type == OBJT_DEVICE || object->type == OBJT_SG) {
+			pmap_object_init_pt(map->pmap, addr, object, pindex,
+			    size);
+			VM_OBJECT_WUNLOCK(object);
+			return;
+		}
+		VM_OBJECT_LOCK_DOWNGRADE(object);
 	}
 
 	psize = atop(size);
 	if (psize > MAX_INIT_PT && (flags & MAP_PREFAULT_PARTIAL) != 0)
 		psize = MAX_INIT_PT;
 	if (psize + pindex > object->size) {
-		if (object->size < pindex)
-			goto unlock_return;
+		if (object->size < pindex) {
+			VM_OBJECT_RUNLOCK(object);
+			return;
+		}
 		psize = object->size - pindex;
 	}
 
@@ -1856,8 +1865,7 @@ vm_map_pmap_enter(vm_map_t map, vm_offset_t addr, vm_prot_t prot,
 	if (p_start != NULL)
 		pmap_enter_object(map->pmap, start, addr + ptoa(psize),
 		    p_start, prot);
-unlock_return:
-	VM_OBJECT_WUNLOCK(object);
+	VM_OBJECT_RUNLOCK(object);
 }
 
 /*
diff --git a/sys/vm/vm_object.h b/sys/vm/vm_object.h
index cf54663..05ab73d 100644
--- a/sys/vm/vm_object.h
+++ b/sys/vm/vm_object.h
@@ -223,6 +223,8 @@ extern struct vm_object kmem_object_store;
 	rw_assert(&(object)->lock, RA_RLOCKED)
 #define	VM_OBJECT_ASSERT_WLOCKED(object)				\
 	rw_assert(&(object)->lock, RA_WLOCKED)
+#define	VM_OBJECT_LOCK_DOWNGRADE(object)				\
+	rw_downgrade(&(object)->lock)
 #define	VM_OBJECT_RLOCK(object)						\
 	rw_rlock(&(object)->lock)
 #define	VM_OBJECT_RUNLOCK(object)					\
diff --git a/sys/vm/vm_page.c b/sys/vm/vm_page.c
index 637fb0c..15697c9 100644
--- a/sys/vm/vm_page.c
+++ b/sys/vm/vm_page.c
@@ -959,7 +959,7 @@ vm_page_find_least(vm_object_t object, vm_pindex_t pindex)
 {
 	vm_page_t m;
 
-	VM_OBJECT_ASSERT_WLOCKED(object);
+	VM_OBJECT_ASSERT_LOCKED(object);
 	if ((m = TAILQ_FIRST(&object->memq)) != NULL && m->pindex < pindex)
 		m = vm_radix_lookup_ge(&object->rtree, pindex);
 	return (m);