Index: sys/amd64/amd64/pmap.c
===================================================================
--- sys/amd64/amd64/pmap.c
+++ sys/amd64/amd64/pmap.c
@@ -3807,6 +3807,23 @@
  * one or two pages may be held during the wait, only to be released
  * afterwards.  This conservative approach is easily argued to avoid
  * race conditions.
+ *
+ * The page index of the page containing PTE at address va is defined
+ * as follows:
+ * - for page table (last level),
+ *      ptepindex = pmap_pde_pindex(va) = va >> PDRSHIFT,
+ *   in other words, it is just the index of the PDE.
+ * - for page directory page,
+ *      ptepindex = NUPDE (number of userland PD entries) +
+ *          (pmap_pde_index(va) >> NPDEPGSHIFT)
+ *   i.e. index of PDPE is put after the last index of PDE,
+ * - for page directory pointer page,
+ *      ptepindex = NUPDE + NUPDPE + (pmap_pde_index(va) >> (NPDEPGSHIFT +
+ *          NPML4EPGSHIFT),
+ *   i.e. index of pml4e is put after the last index of PDPE.
+ * In other words, it is sequential number of the corresponding paging entry
+ * in the order where all entries of the same height are put together, then
+ * heights are put from deepest to root.
  */
 static vm_page_t
 _pmap_allocpte(pmap_t pmap, vm_pindex_t ptepindex, struct rwlock **lockp)