Index: head/sys/kern/kern_sendfile.c
===================================================================
--- head/sys/kern/kern_sendfile.c
+++ head/sys/kern/kern_sendfile.c
@@ -261,7 +261,7 @@
 
 	for (int i = 0; i < count; i++)
 		if (pg[i] != bogus_page)
-			vm_page_xunbusy(pg[i]);
+			vm_page_xunbusy_unchecked(pg[i]);
 
 	if (error)
 		sfio->error = error;
Index: head/sys/vm/vm_page.h
===================================================================
--- head/sys/vm/vm_page.h
+++ head/sys/vm/vm_page.h
@@ -307,7 +307,13 @@
 #define	VPB_SHARERS_WORD(x)	((x) << VPB_SHARERS_SHIFT | VPB_BIT_SHARED)
 #define	VPB_ONE_SHARER		(1 << VPB_SHARERS_SHIFT)
 
-#define	VPB_SINGLE_EXCLUSIVER	VPB_BIT_EXCLUSIVE
+#define	VPB_SINGLE_EXCLUSIVE	VPB_BIT_EXCLUSIVE
+#ifdef INVARIANTS
+#define	VPB_CURTHREAD_EXCLUSIVE						\
+	(VPB_BIT_EXCLUSIVE | ((u_int)(uintptr_t)curthread & ~VPB_BIT_FLAGMASK))
+#else
+#define	VPB_CURTHREAD_EXCLUSIVE	VPB_SINGLE_EXCLUSIVE
+#endif
 
 #define	VPB_UNBUSIED		VPB_SHARERS_WORD(0)
 
@@ -649,6 +655,7 @@
 void vm_page_wire(vm_page_t);
 bool vm_page_wire_mapped(vm_page_t m);
 void vm_page_xunbusy_hard(vm_page_t m);
+void vm_page_xunbusy_hard_unchecked(vm_page_t m);
 void vm_page_set_validclean (vm_page_t, int, int);
 void vm_page_clear_dirty(vm_page_t, int, int);
 void vm_page_set_invalid(vm_page_t, int, int);
@@ -684,10 +691,19 @@
 	    ("vm_page_assert_unbusied: page %p busy @ %s:%d",		\
 	    (m), __FILE__, __LINE__))
 
-#define	vm_page_assert_xbusied(m)					\
+#define	vm_page_assert_xbusied_unchecked(m) do {			\
 	KASSERT(vm_page_xbusied(m),					\
 	    ("vm_page_assert_xbusied: page %p not exclusive busy @ %s:%d", \
-	    (m), __FILE__, __LINE__))
+	    (m), __FILE__, __LINE__));					\
+} while (0)
+#define	vm_page_assert_xbusied(m) do {					\
+	vm_page_assert_xbusied_unchecked(m);				\
+	KASSERT((m->busy_lock & ~VPB_BIT_WAITERS) == 			\
+	    VPB_CURTHREAD_EXCLUSIVE,					\
+	    ("vm_page_assert_xbusied: page %p busy_lock %#x not owned"	\
+            " by me @ %s:%d",						\
+	    (m), (m)->busy_lock, __FILE__, __LINE__));			\
+} while (0)
 
 #define	vm_page_busied(m)						\
 	((m)->busy_lock != VPB_UNBUSIED)
@@ -699,7 +715,7 @@
 } while (0)
 
 #define	vm_page_xbusied(m)						\
-	(((m)->busy_lock & VPB_SINGLE_EXCLUSIVER) != 0)
+	(((m)->busy_lock & VPB_SINGLE_EXCLUSIVE) != 0)
 
 #define	vm_page_xbusy(m) do {						\
 	if (!vm_page_tryxbusy(m))					\
@@ -710,8 +726,13 @@
 /* Note: page m's lock must not be owned by the caller. */
 #define	vm_page_xunbusy(m) do {						\
 	if (!atomic_cmpset_rel_int(&(m)->busy_lock,			\
-	    VPB_SINGLE_EXCLUSIVER, VPB_UNBUSIED))			\
+	    VPB_CURTHREAD_EXCLUSIVE, VPB_UNBUSIED))			\
 		vm_page_xunbusy_hard(m);				\
+} while (0)
+#define	vm_page_xunbusy_unchecked(m) do {				\
+	if (!atomic_cmpset_rel_int(&(m)->busy_lock,			\
+	    VPB_CURTHREAD_EXCLUSIVE, VPB_UNBUSIED))			\
+		vm_page_xunbusy_hard_unchecked(m);			\
 } while (0)
 
 #ifdef INVARIANTS
Index: head/sys/vm/vm_page.c
===================================================================
--- head/sys/vm/vm_page.c
+++ head/sys/vm/vm_page.c
@@ -437,7 +437,7 @@
 	bzero(marker, sizeof(*marker));
 	marker->flags = PG_MARKER;
 	marker->aflags = aflags;
-	marker->busy_lock = VPB_SINGLE_EXCLUSIVER;
+	marker->busy_lock = VPB_CURTHREAD_EXCLUSIVE;
 	marker->queue = queue;
 }
 
@@ -939,18 +939,19 @@
 int
 vm_page_busy_tryupgrade(vm_page_t m)
 {
-	u_int x;
+	u_int ce, x;
 
 	vm_page_assert_sbusied(m);
 
 	x = m->busy_lock;
+	ce = VPB_CURTHREAD_EXCLUSIVE;
 	for (;;) {
 		if (VPB_SHARERS(x) > 1)
 			return (0);
 		KASSERT((x & ~VPB_BIT_WAITERS) == VPB_SHARERS_WORD(1),
 		    ("vm_page_busy_tryupgrade: invalid lock state"));
 		if (!atomic_fcmpset_acq_int(&m->busy_lock, &x,
-		    VPB_SINGLE_EXCLUSIVER | (x & VPB_BIT_WAITERS)))
+		    ce | (x & VPB_BIT_WAITERS)))
 			continue;
 		return (1);
 	}
@@ -1108,7 +1109,7 @@
 	vm_object_t obj;
 
         if (atomic_cmpset_acq_int(&(m)->busy_lock, VPB_UNBUSIED,
-            VPB_SINGLE_EXCLUSIVER) == 0)
+            VPB_CURTHREAD_EXCLUSIVE) == 0)
 		return (0);
 
 	obj = m->object;
@@ -1119,6 +1120,14 @@
 	return (1);
 }
 
+static void
+vm_page_xunbusy_hard_tail(vm_page_t m)
+{
+	atomic_store_rel_int(&m->busy_lock, VPB_UNBUSIED);
+	/* Wake the waiter. */
+	wakeup(m);
+}
+
 /*
  *	vm_page_xunbusy_hard:
  *
@@ -1127,14 +1136,15 @@
 void
 vm_page_xunbusy_hard(vm_page_t m)
 {
-
 	vm_page_assert_xbusied(m);
+	vm_page_xunbusy_hard_tail(m);
+}
 
-	/*
-	 * Wake the waiter.
-	 */
-	atomic_store_rel_int(&m->busy_lock, VPB_UNBUSIED);
-	wakeup(m);
+void
+vm_page_xunbusy_hard_unchecked(vm_page_t m)
+{
+	vm_page_assert_xbusied_unchecked(m);
+	vm_page_xunbusy_hard_tail(m);
 }
 
 /*
@@ -1228,7 +1238,7 @@
 	m->flags = PG_FICTITIOUS;
 	/* Fictitious pages don't use "order" or "pool". */
 	m->oflags = VPO_UNMANAGED;
-	m->busy_lock = VPB_SINGLE_EXCLUSIVER;
+	m->busy_lock = VPB_CURTHREAD_EXCLUSIVE;
 	/* Fictitious pages are unevictable. */
 	m->ref_count = 1;
 	pmap_page_init(m);
@@ -1318,7 +1328,7 @@
 	else
 		vm_page_deactivate(m);
 	vm_page_unlock(m);
-	vm_page_xunbusy(m);
+	vm_page_xunbusy_unchecked(m);
 }
 
 /*
@@ -1967,7 +1977,7 @@
 	    VPO_UNMANAGED : 0;
 	m->busy_lock = VPB_UNBUSIED;
 	if ((req & (VM_ALLOC_NOBUSY | VM_ALLOC_NOOBJ | VM_ALLOC_SBUSY)) == 0)
-		m->busy_lock = VPB_SINGLE_EXCLUSIVER;
+		m->busy_lock = VPB_CURTHREAD_EXCLUSIVE;
 	if ((req & VM_ALLOC_SBUSY) != 0)
 		m->busy_lock = VPB_SHARERS_WORD(1);
 	if (req & VM_ALLOC_WIRED) {
@@ -2161,7 +2171,7 @@
 	    VPO_UNMANAGED : 0;
 	busy_lock = VPB_UNBUSIED;
 	if ((req & (VM_ALLOC_NOBUSY | VM_ALLOC_NOOBJ | VM_ALLOC_SBUSY)) == 0)
-		busy_lock = VPB_SINGLE_EXCLUSIVER;
+		busy_lock = VPB_CURTHREAD_EXCLUSIVE;
 	if ((req & VM_ALLOC_SBUSY) != 0)
 		busy_lock = VPB_SHARERS_WORD(1);
 	if ((req & VM_ALLOC_WIRED) != 0)