Index: sys/kern/subr_physmem.c
===================================================================
--- sys/kern/subr_physmem.c
+++ sys/kern/subr_physmem.c
@@ -278,19 +278,32 @@
     vm_size_t size, uint32_t flags)
 {
 	size_t i;
+	vm_paddr_t nend, rend;
 	struct region *ep, *rp;
 
+	nend = addr + size;
 	ep = regions + rcnt;
 	for (i = 0, rp = regions; i < rcnt; ++i, ++rp) {
-		if (rp->addr == addr && rp->size == size) /* Pure dup. */
-			return (rcnt);
 		if (flags == rp->flags) {
-			if (addr + size == rp->addr) {
+			rend = rp->addr + rp->size;
+			if (addr <= rp->addr && nend >= rp->addr) {
+				/*
+				 * New mapping overlaps at the beginning, shift
+				 * for any difference in the beginning then
+				 * shift if the new mapping extends past.
+				 */
+				rp->size += rp->addr - addr;
 				rp->addr = addr;
-				rp->size += size;
+				if (nend >= rend)
+					rp->size += nend - rend;
 				return (rcnt);
-			} else if (rp->addr + rp->size == addr) {
-				rp->size += size;
+			} else if (addr <= rend && nend > rp->addr) {
+				/*
+				 * New mapping is either entirely contained
+				 * within or it's overlapping at the end.
+				 */
+				if (nend > rend)
+					rp->size += nend - rend;
 				return (rcnt);
 			}
 		}