i've noticed that extent_alloc() can fail and return a value outside
of the extent when there the map start address + the boundary condition
overflow.  this happens on the sparc64 with the sbus dvma map (which
is mapped near the very top of kva) when mapping the le driver (which
has a boundary condition of 16M).  the follow patch fixes this problem
by noticing the overflow when checking boundary condition..


Index: subr_extent.c
===================================================================
RCS file: /cvsroot/syssrc/sys/kern/subr_extent.c,v
retrieving revision 1.31
diff -p -r1.31 subr_extent.c
*** subr_extent.c	2000/06/06 17:32:50	1.31
--- subr_extent.c	2000/06/16 06:02:21
*************** extent_alloc_subregion1(ex, substart, su
*** 673,679 ****
  				    boundary, dontcross);
  #endif
  
! 				if (newend > dontcross) {
  					/*
  					 * Candidate region crosses boundary.
  					 * Throw away the leading part and see
--- 673,682 ----
  				    boundary, dontcross);
  #endif
  
! 				/* Check for overflow */
! 				if (dontcross < ex->ex_start)
! 					dontcross = ex->ex_end;
! 				else if (newend > dontcross) {
  					/*
  					 * Candidate region crosses boundary.
  					 * Throw away the leading part and see
*************** extent_alloc_subregion1(ex, substart, su
*** 764,770 ****
  			    boundary, dontcross);
  #endif
  
! 			if (newend > dontcross) {
  				/*
  				 * Candidate region crosses boundary.
  				 * Throw away the leading part and see
--- 767,776 ----
  			    boundary, dontcross);
  #endif
  
! 			/* Check for overflow */
! 			if (dontcross < ex->ex_start)
! 				dontcross = ex->ex_end;
! 			else if (newend > dontcross) {
  				/*
  				 * Candidate region crosses boundary.
  				 * Throw away the leading part and see