Add a return value to vm_page_remove().

Use it to indicate whether the page may be safely freed following
its removal from the object.  Also change vm_page_remove() to assume
that the page's object pointer is non-NULL, and have callers perform
this check instead.

This is a step towards an implementation of an atomic reference counter
for each physical page structure.

Reviewed by:	alc, dougm, kib
MFC after:	1 week
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D20758

sys/amd64/sgx/sgx.c

@@ -358,7 +358,7 @@ sgx_page_remove(struct sgx_softc *sc, vm_page_t p)
 	uint64_t offs;
 
 	vm_page_lock(p);
-	vm_page_remove(p);
+	(void)vm_page_remove(p);
 	vm_page_unlock(p);
 
 	dprintf("%s: p->pidx %ld\n", __func__, p->pindex);

sys/dev/drm2/ttm/ttm_bo_vm.c

@@ -115,7 +115,7 @@ ttm_bo_vm_fault(vm_object_t vm_obj, vm_ooffset_t offset,
 	vm_object_pip_add(vm_obj, 1);
 	if (*mres != NULL) {
 		vm_page_lock(*mres);
-		vm_page_remove(*mres);
+		(void)vm_page_remove(*mres);
 		vm_page_unlock(*mres);
 	}
 retry:

sys/vm/device_pager.c

@@ -236,7 +236,7 @@ cdev_pager_free_page(vm_object_t object, vm_page_t m)
 		KASSERT((m->oflags & VPO_UNMANAGED) == 0, ("unmanaged %p", m));
 		pmap_remove_all(m);
 		vm_page_lock(m);
-		vm_page_remove(m);
+		(void)vm_page_remove(m);
 		vm_page_unlock(m);
 	} else if (object->type == OBJT_DEVICE)
 		dev_pager_free_page(object, m);

sys/vm/vm_fault.c

@@ -1144,7 +1144,7 @@ RetryFault:;
 			    fs.object == fs.first_object->backing_object) {
 				vm_page_lock(fs.m);
 				vm_page_dequeue(fs.m);
-				vm_page_remove(fs.m);
+				(void)vm_page_remove(fs.m);
 				vm_page_unlock(fs.m);
 				vm_page_lock(fs.first_m);
 				vm_page_replace_checked(fs.m, fs.first_object,

sys/vm/vm_object.c

@@ -1595,10 +1595,8 @@ vm_object_collapse_scan(vm_object_t object, int op)
 			vm_page_lock(p);
 			KASSERT(!pmap_page_is_mapped(p),
 			    ("freeing mapped page %p", p));
-			if (!vm_page_wired(p))
+			if (vm_page_remove(p))
 				vm_page_free(p);
-			else
-				vm_page_remove(p);
 			vm_page_unlock(p);
 			continue;
 		}
@@ -1639,10 +1637,8 @@ vm_object_collapse_scan(vm_object_t object, int op)
 			vm_page_lock(p);
 			KASSERT(!pmap_page_is_mapped(p),
 			    ("freeing mapped page %p", p));
-			if (!vm_page_wired(p))
+			if (vm_page_remove(p))
 				vm_page_free(p);
-			else
-				vm_page_remove(p);
 			vm_page_unlock(p);
 			continue;
 		}

sys/vm/vm_page.c

@@ -1458,20 +1458,21 @@ vm_page_insert_radixdone(vm_page_t m, vm_object_t object, vm_page_t mpred)
  *	vm_page_remove:
  *
  *	Removes the specified page from its containing object, but does not
- *	invalidate any backing storage.
+ *	invalidate any backing storage.  Return true if the page may be safely
+ *	freed and false otherwise.
  *
  *	The object must be locked.  The page must be locked if it is managed.
  */
-void
+bool
 vm_page_remove(vm_page_t m)
 {
 	vm_object_t object;
 	vm_page_t mrem;
 
+	object = m->object;
+
 	if ((m->oflags & VPO_UNMANAGED) == 0)
 		vm_page_assert_locked(m);
-	if ((object = m->object) == NULL)
-		return;
 	VM_OBJECT_ASSERT_WLOCKED(object);
 	if (vm_page_xbusied(m))
 		vm_page_xunbusy_maybelocked(m);
@@ -1495,6 +1496,7 @@ vm_page_remove(vm_page_t m)
 		vdrop(object->handle);
 
 	m->object = NULL;
+	return (!vm_page_wired(m));
 }
 
 /*
@@ -1665,7 +1667,7 @@ vm_page_rename(vm_page_t m, vm_object_t new_object, vm_pindex_t new_pindex)
 	 */
 	m->pindex = opidx;
 	vm_page_lock(m);
-	vm_page_remove(m);
+	(void)vm_page_remove(m);
 
 	/* Return back to the new pindex to complete vm_page_insert(). */
 	m->pindex = new_pindex;
@@ -3436,7 +3438,8 @@ vm_page_free_prep(vm_page_t m)
 	if (vm_page_sbusied(m))
 		panic("vm_page_free_prep: freeing busy page %p", m);
 
-	vm_page_remove(m);
+	if (m->object != NULL)
+		(void)vm_page_remove(m);
 
 	/*
 	 * If fictitious remove object association and

sys/vm/vm_page.h

@@ -561,7 +561,7 @@ bool vm_page_reclaim_contig(int req, u_long npages, vm_paddr_t low,
 bool vm_page_reclaim_contig_domain(int domain, int req, u_long npages,
     vm_paddr_t low, vm_paddr_t high, u_long alignment, vm_paddr_t boundary);
 void vm_page_reference(vm_page_t m);
-void vm_page_remove (vm_page_t);
+bool vm_page_remove(vm_page_t);
 int vm_page_rename (vm_page_t, vm_object_t, vm_pindex_t);
 vm_page_t vm_page_replace(vm_page_t mnew, vm_object_t object,
     vm_pindex_t pindex);