Index: sys/kern/kern_dump.c
===================================================================
--- sys/kern/kern_dump.c
+++ sys/kern/kern_dump.c
@@ -31,8 +31,10 @@
 #include <sys/systm.h>
 #include <sys/conf.h>
 #include <sys/cons.h>
+#include <sys/kdb.h>
 #include <sys/kernel.h>
 #include <sys/kerneldump.h>
+#include <sys/malloc.h>
 #include <sys/msgbuf.h>
 #include <sys/proc.h>
 #include <sys/watchdog.h>
@@ -295,7 +297,7 @@
 
 #if MINIDUMP_PAGE_TRACKING == 1
 	if (do_minidump)
-		return (minidumpsys(di));
+		return (minidumpsys(di, false));
 #endif
 
 	bzero(&ehdr, sizeof(ehdr));
@@ -461,15 +463,41 @@
 }
 
 int
-minidumpsys(struct dumperinfo *di)
+minidumpsys(struct dumperinfo *di, bool livedump)
 {
 	struct minidumpstate state;
+	struct msgbuf mb_copy;
+	char *msg_ptr;
+	size_t sz;
 	int error;
 
-	state.msgbufp = msgbufp;
-	state.dump_bitset = vm_page_dump;
+	if (livedump) {
+		KASSERT(!kdb_active, ("livedump requested from debugger"));
+
+		/*
+		 * Snapshot the required global state: message buffer, page
+		 * dump bitset. They may change during the live dump.
+		 */
+		msg_ptr = malloc(msgbufsize, M_TEMP, M_WAITOK);
+		msgbuf_duplicate(msgbufp, &mb_copy, msg_ptr);
+		state.msgbufp = &mb_copy;
+
+		sz = BITSET_SIZE(vm_page_dump_pages);
+		state.dump_bitset = malloc(sz, M_TEMP, M_WAITOK);
+		BIT_COPY_STORE_REL(sz, vm_page_dump, state.dump_bitset);
+	} else {
+		KASSERT(kdb_active, ("minidump invoked outside of debugger"));
+
+		/* Use the globals. */
+		state.msgbufp = msgbufp;
+		state.dump_bitset = vm_page_dump;
+	}
 
 	error = cpu_minidumpsys(di, &state);
+	if (livedump) {
+		free(msg_ptr, M_TEMP);
+		free(state.dump_bitset, M_TEMP);
+	}
 
 	return (error);
 }
Index: sys/kern/subr_msgbuf.c
===================================================================
--- sys/kern/subr_msgbuf.c
+++ sys/kern/subr_msgbuf.c
@@ -414,3 +414,18 @@
 	while ((c = msgbuf_getchar(src)) >= 0)
 		msgbuf_addchar(dst, c);
 }
+
+/*
+ * Get a snapshot of the message buffer, without modifying its internal state
+ * (i.e. don't mark any new characters as read).
+ */
+void
+msgbuf_duplicate(struct msgbuf *src, struct msgbuf *dst, char *dst_msgptr)
+{
+
+	mtx_lock_spin(&src->msg_lock);
+	bcopy(src, dst, sizeof(struct msgbuf));
+	dst->msg_ptr = dst_msgptr;
+	bcopy(src->msg_ptr, dst->msg_ptr, src->msg_size);
+	mtx_unlock_spin(&src->msg_lock);
+}
Index: sys/sys/kerneldump.h
===================================================================
--- sys/sys/kerneldump.h
+++ sys/sys/kerneldump.h
@@ -140,7 +140,7 @@
 	struct bitset	*dump_bitset;
 };
 
-int minidumpsys(struct dumperinfo *);
+int minidumpsys(struct dumperinfo *, bool);
 int dumpsys_generic(struct dumperinfo *);
 
 void dumpsys_map_chunk(vm_paddr_t, size_t, void **);
Index: sys/sys/msgbuf.h
===================================================================
--- sys/sys/msgbuf.h
+++ sys/sys/msgbuf.h
@@ -78,6 +78,7 @@
 int	msgbuf_peekbytes(struct msgbuf *mbp, char *buf, int buflen,
 	    u_int *seqp);
 void	msgbuf_reinit(struct msgbuf *mbp, void *ptr, int size);
+void	msgbuf_duplicate(struct msgbuf *src, struct msgbuf *dst, char *msgptr);
 
 #ifndef MSGBUF_SIZE
 #define	MSGBUF_SIZE	(32768 * 3)