sysctl vm.vm_objects: report cdev name for device-backed objects

Reviewed by:	emaste. markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D47038

sys/vm/vm_object.c

@@ -66,6 +66,7 @@
 
 #include <sys/systm.h>
 #include <sys/blockcount.h>
+#include <sys/conf.h>
 #include <sys/cpuset.h>
 #include <sys/ipc.h>
 #include <sys/jail.h>
@@ -2506,8 +2507,10 @@ vm_object_list_handler(struct sysctl_req *req, bool swap_only)
 	struct vattr va;
 	vm_object_t obj;
 	vm_page_t m;
+	struct cdev *cdev;
+	struct cdevsw *csw;
 	u_long sp;
-	int count, error;
+	int count, error, ref;
 	key_t key;
 	unsigned short seq;
 	bool want_path;
@@ -2594,6 +2597,17 @@ vm_object_list_handler(struct sysctl_req *req, bool swap_only)
 			sp = swap_pager_swapped_pages(obj);
 			kvo->kvo_swapped = sp > UINT32_MAX ? UINT32_MAX : sp;
 		}
+		if (obj->type == OBJT_DEVICE || obj->type == OBJT_MGTDEVICE) {
+			cdev = obj->un_pager.devp.dev;
+			if (cdev != NULL) {
+				csw = dev_refthread(cdev, &ref);
+				if (csw != NULL) {
+					strlcpy(kvo->kvo_path, cdev->si_name,
+					    sizeof(kvo->kvo_path));
+					dev_relthread(cdev, ref);
+				}
+			}
+		}
 		VM_OBJECT_RUNLOCK(obj);
 		if ((obj->flags & OBJ_SYSVSHM) != 0) {
 			kvo->kvo_flags |= KVMO_FLAG_SYSVSHM;