PVN_GETPAGE_{SZ,NUM} are misnamed and unnecessarily complicated
There is really no reason to not allow 8 pages all the time.  With the
current logic, we get the following:
Assuming 4kB pages (x86):
    _SZ  = ptob(8) /* 32kB */
    _NUM = 8
Assuming 8kB pages (sparc):
    _SZ  = ptob(8) /* 64kB */
    _NUM = 8
We'd have to deal with 16kB base pages in order for the _NUM #define to not
be 8 (it'd be 4 in that case).  So, in the spirit of simplicity, let's just
always grab 8 pages as there are no interesting systems with 16kB+ base pages.
Finally, the defines are poorly named.

*** 76,102 ****
  #include <sys/zone.h>
  #include <sys/shm_impl.h>
  
  /*
   * segvn_fault needs a temporary page list array.  To avoid calling kmem all
!  * the time, it creates a small (PVN_GETPAGE_NUM entry) array and uses it if
!  * it can.  In the rare case when this page list is not large enough, it
!  * goes and gets a large enough array from kmem.
!  *
!  * This small page list array covers either 8 pages or 64kB worth of pages -
!  * whichever is smaller.
   */
! #define PVN_MAX_GETPAGE_SZ      0x10000
! #define PVN_MAX_GETPAGE_NUM     0x8
! 
! #if PVN_MAX_GETPAGE_SZ > PVN_MAX_GETPAGE_NUM * PAGESIZE
! #define PVN_GETPAGE_SZ  ptob(PVN_MAX_GETPAGE_NUM)
! #define PVN_GETPAGE_NUM PVN_MAX_GETPAGE_NUM
! #else
! #define PVN_GETPAGE_SZ  PVN_MAX_GETPAGE_SZ
! #define PVN_GETPAGE_NUM btop(PVN_MAX_GETPAGE_SZ)
! #endif
  
  /*
   * Private seg op routines.
   */
  static int      segvn_dup(struct seg *seg, struct seg *newseg);
--- 76,91 ----
  #include <sys/zone.h>
  #include <sys/shm_impl.h>
  
  /*
   * segvn_fault needs a temporary page list array.  To avoid calling kmem all
!  * the time, it creates a small (FAULT_TMP_PAGES_NUM entry) array and uses
!  * it if it can.  In the rare case when this page list is not large enough,
!  * it goes and gets a large enough array from kmem.
   */
! #define FAULT_TMP_PAGES_NUM     0x8
! #define FAULT_TMP_PAGES_SZ      ptob(FAULT_TMP_PAGES_NUM)
  
  /*
   * Private seg op routines.
   */
  static int      segvn_dup(struct seg *seg, struct seg *newseg);
*** 4923,4933 ****
          u_offset_t off;
          caddr_t a;
          struct vpage *vpage;
          uint_t vpprot, prot;
          int err;
!         page_t *pl[PVN_GETPAGE_NUM + 1];
          size_t plsz, pl_alloc_sz;
          size_t page;
          ulong_t anon_index;
          struct anon_map *amp;
          int dogetpage = 0;
--- 4912,4922 ----
          u_offset_t off;
          caddr_t a;
          struct vpage *vpage;
          uint_t vpprot, prot;
          int err;
!         page_t *pl[FAULT_TMP_PAGES_NUM + 1];
          size_t plsz, pl_alloc_sz;
          size_t page;
          ulong_t anon_index;
          struct anon_map *amp;
          int dogetpage = 0;
*** 5362,5372 ****
  
                  if (dogetpage) {
                          enum seg_rw arw;
                          struct as *as = seg->s_as;
  
!                         if (len > ptob((sizeof (pl) / sizeof (pl[0])) - 1)) {
                                  /*
                                   * Page list won't fit in local array,
                                   * allocate one of the needed size.
                                   */
                                  pl_alloc_sz =
--- 5351,5361 ----
  
                  if (dogetpage) {
                          enum seg_rw arw;
                          struct as *as = seg->s_as;
  
!                         if (len > FAULT_TMP_PAGES_SZ) {
                                  /*
                                   * Page list won't fit in local array,
                                   * allocate one of the needed size.
                                   */
                                  pl_alloc_sz =
*** 5390,5400 ****
                          } else {
                                  /*
                                   * Ask VOP_GETPAGE to return adjacent pages
                                   * within the segment.
                                   */
!                                 plsz = MIN((size_t)PVN_GETPAGE_SZ, (size_t)
                                      ((seg->s_base + seg->s_size) - addr));
                                  ASSERT((addr + plsz) <=
                                      (seg->s_base + seg->s_size));
                          }
  
--- 5379,5389 ----
                          } else {
                                  /*
                                   * Ask VOP_GETPAGE to return adjacent pages
                                   * within the segment.
                                   */
!                                 plsz = MIN((size_t)FAULT_TMP_PAGES_SZ, (size_t)
                                      ((seg->s_base + seg->s_size) - addr));
                                  ASSERT((addr + plsz) <=
                                      (seg->s_base + seg->s_size));
                          }