svn commit: r364286 - head/sys/vm

[Konstantin Belousov]

Author: kib
Date: Sun Aug 16 20:57:02 2020
New Revision: 364286
URL: https://svnweb.freebsd.org/changeset/base/364286

Log:
  vm_object: allow paging_in_progress to be acquired after object termination.
  
  The vm objects are type-stable, and can be accessed even after the
  last reference is dropped, or in case of vnode objects, after vgone()
  destroyed it as well.
  
  Stop asserting that pip == 0 after vm_object_terminate() waited for
  existing owners to drop it, we only want to drain them before setting
  OBJ_DEAD flag.  Also stop asserting pip == 0 in object destructor.
  
  Update comments explaining the interaction between paging_in_progress
  and termination.
  
  Reviewed by:	markj
  Tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  Differential revision:	https://reviews.freebsd.org/D25968

Modified:
  head/sys/vm/vm_object.c

Modified: head/sys/vm/vm_object.c
==============================================================================
--- head/sys/vm/vm_object.c	Sun Aug 16 20:52:24 2020	(r364285)
+++ head/sys/vm/vm_object.c	Sun Aug 16 20:57:02 2020	(r364286)
@@ -192,9 +192,6 @@ vm_object_zdtor(void *mem, int size, void *arg)
 	    ("object %p has reservations",
 	    object));
 #endif
-	KASSERT(blockcount_read(&object->paging_in_progress) == 0,
-	    ("object %p paging_in_progress = %d",
-	    object, blockcount_read(&object->paging_in_progress)));
 	KASSERT(!vm_object_busied(object),
 	    ("object %p busy = %d", object, blockcount_read(&object->busy)));
 	KASSERT(object->resident_page_count == 0,
@@ -294,6 +291,9 @@ vm_object_init(void)
 	 * The lock portion of struct vm_object must be type stable due
 	 * to vm_pageout_fallback_object_lock locking a vm object
 	 * without holding any references to it.
+	 *
+	 * paging_in_progress is valid always.  Lockless references to
+	 * the objects may acquire pip and then check OBJ_DEAD.
 	 */
 	obj_zone = uma_zcreate("VM OBJECT", sizeof (struct vm_object), NULL,
 #ifdef INVARIANTS
@@ -936,12 +936,13 @@ vm_object_terminate(vm_object_t object)
 	    ("terminating shadow obj %p", object));
 
 	/*
-	 * wait for the pageout daemon to be done with the object
+	 * Wait for the pageout daemon and other current users to be
+	 * done with the object.  Note that new paging_in_progress
+	 * users can come after this wait, but they must check
+	 * OBJ_DEAD flag set (without unlocking the object), and avoid
+	 * the object being terminated.
 	 */
 	vm_object_pip_wait(object, "objtrm");
-
-	KASSERT(!blockcount_read(&object->paging_in_progress),
-	    ("vm_object_terminate: pageout in progress"));
 
 	KASSERT(object->ref_count == 0,
 	    ("vm_object_terminate: object with references, ref_count=%d",