Index: sys/vm/vm_object.h
===================================================================
--- sys/vm/vm_object.h	(revisione 237271)
+++ sys/vm/vm_object.h	(copia locale)
@@ -76,8 +76,22 @@
  *
  *	vm_object_t		Virtual memory object.
  *
+ *	The root of cached pages pool is protected by both the per-object mutex
+ *	and the free pages queue mutex.
+ *	On insert in the cache splay tree, the per-object mutex is expected
+ *	to be already held and the free paging queue mutex will be
+ *	acquired during the operation too.
+ *	On remove and lookup from the cache splay tree, only the free
+ *	paging queue mutex is expected to be locked.
+ *	These rules allow to check reliably the presence of cached pages
+ *	with only the per-mutex lock held, avoiding to acquire the
+ *	free paging queue mutex unnecessarilly and thus reducing the
+ *	contention on it.
+ *
  * List of locks
  *	(c)	const until freed
+ *	(o)	per-object mutex
+ *	(q)	free paging queue mutex
  *
  */
 
@@ -102,7 +116,7 @@ struct vm_object {
 	vm_ooffset_t backing_object_offset;/* Offset in backing object */
 	TAILQ_ENTRY(vm_object) pager_object_list; /* list of all objects of this pager type */
 	LIST_HEAD(, vm_reserv) rvq;	/* list of reservations */
-	vm_page_t cache;		/* root of the cache page splay tree */
+	vm_page_t cache;		/* (o + q) root of the cache page splay tree */
 	void *handle;
 	union {
 		/*