Index: sys/kern/subr_extent.c
===================================================================
RCS file: /cvsroot/src/sys/kern/subr_extent.c,v
retrieving revision 1.76
diff -p -u -r1.76 subr_extent.c
--- sys/kern/subr_extent.c	10 Jul 2014 21:13:52 -0000	1.76
+++ sys/kern/subr_extent.c	5 Apr 2015 12:09:48 -0000
@@ -600,6 +600,7 @@ extent_alloc_region(struct extent *ex, u
  * The allocated region is aligned to "alignment", which must be
  * a power of 2.
  */
+#include <ddb/ddb.h>
 int
 extent_alloc_subregion1(struct extent *ex, u_long substart, u_long subend,
     u_long size, u_long alignment, u_long skew, u_long boundary,
@@ -968,6 +969,12 @@ skip:
 	 */
 	extent_insert_and_optimize(ex, newstart, size, flags, last, myrp);
 	mutex_exit(&ex->ex_lock);
+	if (newstart <= 0xc0000 && 0xc0000 - newstart < size) {
+		printf("%s: extent allocing vga rom: [%lu, %lu)",
+		    __func__, newstart, newstart + size);
+		db_stack_trace_print((db_expr_t)(intptr_t)
+		    __builtin_frame_address(0), true, 65536, "", printf);
+	}
 	*result = newstart;
 	return (0);
 }