From: Martin Pieuchot <mpi@grenadille.net>
Subject: uvm_purge()
To: tech@openbsd.org
Date: Mon, 12 May 2025 20:42:40 +0200

Diff below moves the tearing down of VM space to exit1().  It implies
that processes will now be charged for cleaning their VM space.  As a
result their %sys time will increase.

uvm_purge() is called in the "top" part of exit1() when the process is
still allowed to sleep.  To allow this piece of code to be executed in
parallel, all recursive kernel_lock tokens need to be released. 

In the diff below uvm_purge() is called twice.  The first time in
exit1() and the second time, doing nothing but integrity checks, in
uvm_exit().  Another approach could be to just check that the map is
empty.  Any preferences?

Comments?  Oks?

Index: kern/kern_exit.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_exit.c,v
diff -u -p -r1.245 kern_exit.c
--- kern/kern_exit.c	2 May 2025 05:04:38 -0000	1.245
+++ kern/kern_exit.c	12 May 2025 18:10:35 -0000
@@ -241,6 +241,14 @@ exit1(struct proc *p, int xexit, int xsi
 		 */
 		if (pr->ps_pptr->ps_sigacts->ps_sigflags & SAS_NOCLDWAIT)
 			atomic_setbits_int(&pr->ps_flags, PS_NOZOMBIE);
+
+		/* Teardown the virtual address space. */
+#ifdef MULTIPROCESSOR
+		__mp_release_all(&kernel_lock);
+#endif
+		uvm_purge(pr);
+		KERNEL_LOCK();
+
 	}
 
 	p->p_fd = NULL;		/* zap the thread's copy */
Index: uvm/uvm_extern.h
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_extern.h,v
diff -u -p -r1.180 uvm_extern.h
--- uvm/uvm_extern.h	19 Nov 2024 06:18:26 -0000	1.180
+++ uvm/uvm_extern.h	12 May 2025 18:05:34 -0000
@@ -268,6 +268,7 @@ int			uvm_fault(vm_map_t, vaddr_t, vm_fa
 
 vaddr_t			uvm_uarea_alloc(void);
 void			uvm_uarea_free(struct proc *);
+void			uvm_purge(struct process *);
 void			uvm_exit(struct process *);
 void			uvm_init_limits(struct plimit *);
 boolean_t		uvm_kernacc(caddr_t, size_t, int);
@@ -401,6 +402,7 @@ void			uvmspace_init(struct vmspace *, s
 void			uvmspace_exec(struct proc *, vaddr_t, vaddr_t);
 struct vmspace		*uvmspace_fork(struct process *);
 void			uvmspace_addref(struct vmspace *);
+void			uvmspace_purge(struct vmspace *);
 void			uvmspace_free(struct vmspace *);
 struct vmspace		*uvmspace_share(struct process *);
 int			uvm_share(vm_map_t, vaddr_t, vm_prot_t,
Index: uvm/uvm_glue.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_glue.c,v
diff -u -p -r1.88 uvm_glue.c
--- uvm/uvm_glue.c	21 Mar 2025 13:19:33 -0000	1.88
+++ uvm/uvm_glue.c	12 May 2025 18:05:34 -0000
@@ -286,6 +286,19 @@ uvm_uarea_free(struct proc *p)
 }
 
 /*
+ * uvm_purge: teardown a virtual address space
+ */
+void
+uvm_purge(struct process *pr)
+{
+	struct vmspace *vm = pr->ps_vmspace;
+
+	KERNEL_ASSERT_UNLOCKED();
+
+	uvmspace_purge(vm);
+}
+
+/*
  * uvm_exit: exit a virtual address space
  */
 void
Index: uvm/uvm_map.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_map.c,v
diff -u -p -r1.341 uvm_map.c
--- uvm/uvm_map.c	21 Apr 2025 14:46:18 -0000	1.341
+++ uvm/uvm_map.c	12 May 2025 18:05:34 -0000
@@ -2491,6 +2491,19 @@ uvm_map_teardown(struct vm_map *map)
 		entry = TAILQ_NEXT(entry, dfree.deadq);
 	}
 
+#ifdef VMMAP_DEBUG
+	numt = numq = 0;
+	RBT_FOREACH(entry, uvm_map_addr, &map->addr)
+		numt++;
+	TAILQ_FOREACH(entry, &dead_entries, dfree.deadq)
+		numq++;
+	KASSERT(numt == numq);
+#endif
+	RBT_INIT(uvm_map_addr, &map->addr);
+	map->size = 0;
+	map->min_offset = 0;
+	map->max_offset = 0;
+	map->flags &= ~VM_MAP_ISVMSPACE;
 	vm_map_unlock(map);
 
 	/* Remove address selectors. */
@@ -2503,18 +2516,7 @@ uvm_map_teardown(struct vm_map *map)
 	uvm_addr_destroy(map->uaddr_brk_stack);
 	map->uaddr_brk_stack = NULL;
 
-#ifdef VMMAP_DEBUG
-	numt = numq = 0;
-	RBT_FOREACH(entry, uvm_map_addr, &map->addr)
-		numt++;
-	TAILQ_FOREACH(entry, &dead_entries, dfree.deadq)
-		numq++;
-	KASSERT(numt == numq);
-#endif
 	uvm_unmap_detach(&dead_entries, 0);
-
-	pmap_destroy(map->pmap);
-	map->pmap = NULL;
 }
 
 /*
@@ -3395,6 +3397,21 @@ uvmspace_addref(struct vmspace *vm)
 	atomic_inc_int(&vm->vm_refcnt);
 }
 
+void
+uvmspace_purge(struct vmspace *vm)
+{
+#ifdef SYSVSHM
+	/* Get rid of any SYSV shared memory segments. */
+	if (vm->vm_shm != NULL) {
+		KERNEL_LOCK();
+		shmexit(vm);
+		KERNEL_UNLOCK();
+	}
+#endif
+
+	uvm_map_teardown(&vm->vm_map);
+}
+
 /*
  * uvmspace_free: free a vmspace data structure
  */
@@ -3407,16 +3424,11 @@ uvmspace_free(struct vmspace *vm)
 		 * all of the mappings and pages they hold, then call the pmap
 		 * module to reclaim anything left.
 		 */
-#ifdef SYSVSHM
-		/* Get rid of any SYSV shared memory segments. */
-		if (vm->vm_shm != NULL) {
-			KERNEL_LOCK();
-			shmexit(vm);
-			KERNEL_UNLOCK();
-		}
-#endif
+		uvmspace_purge(vm);
+
+		pmap_destroy(vm->vm_map.pmap);
+		vm->vm_map.pmap = NULL;
 
-		uvm_map_teardown(&vm->vm_map);
 		pool_put(&uvm_vmspace_pool, vm);
 	}
 }