Don't acquire a reference on the VM-space when allocating the LinuxKPI

task structure to avoid deadlock when tearing down the VM object
during a process exit.

Found by:		markj @
MFC after:		1 week
Sponsored by:		Mellanox Technologies

sys/compat/linuxkpi/common/include/linux/mm_types.h

@@ -38,11 +38,9 @@
 
 struct vm_area_struct;
 struct task_struct;
-struct vmspace;
 
 struct mm_struct {
 	struct vm_area_struct *mmap;
-	struct vmspace *vmspace;
 	atomic_t mm_count;
 	atomic_t mm_users;
 	size_t pinned_vm;

sys/compat/linuxkpi/common/src/linux_current.c

@@ -96,7 +96,6 @@ linux_alloc_current(struct thread *td, int flags)
 		init_rwsem(&mm->mmap_sem);
 		atomic_set(&mm->mm_count, 1);
 		atomic_set(&mm->mm_users, 1);
-		mm->vmspace = vmspace_acquire_ref(proc);
 		/* set mm_struct pointer */
 		ts->mm = mm;
 		/* clear pointer to not free memory */
@@ -119,7 +118,7 @@ linux_get_task_mm(struct task_struct *task)
 	struct mm_struct *mm;
 
 	mm = task->mm;
-	if (mm != NULL && mm->vmspace != NULL) {
+	if (mm != NULL) {
 		atomic_inc(&mm->mm_users);
 		return (mm);
 	}
@@ -129,8 +128,6 @@ linux_get_task_mm(struct task_struct *task)
 void
 linux_mm_dtor(struct mm_struct *mm)
 {
-	if (mm->vmspace != NULL)
-		vmspace_free(mm->vmspace);
 	free(mm, M_LINUX_CURRENT);
 }
 

sys/compat/linuxkpi/common/src/linux_page.c

@@ -268,7 +268,7 @@ get_user_pages_remote(struct task_struct *task, struct mm_struct *mm,
 {
 	vm_map_t map;
 
-	map = &mm->vmspace->vm_map;
+	map = &task->task_thread->td_proc->p_vmspace->vm_map;
 	return (linux_get_user_pages_internal(map, start, nr_pages,
 	    !!(gup_flags & FOLL_WRITE), pages));
 }