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Index: head/sys/amd64/amd64/pmap.c
===================================================================
--- head/sys/amd64/amd64/pmap.c
+++ head/sys/amd64/amd64/pmap.c
@@ -3807,6 +3807,29 @@
* 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 ptepindexes, i.e. page indices, of the page table pages encountered
+ * while translating virtual address va are defined as follows:
+ * - for the page table page (last level),
+ * ptepindex = pmap_pde_pindex(va) = va >> PDRSHIFT,
+ * in other words, it is just the index of the PDE that maps the page
+ * table page.
+ * - for the 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 the 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.
+ *
+ * Define an order on the paging entries, where all entries of the
+ * same height are put together, then heights are put from deepest to
+ * root. Then ptexpindex is the sequential number of the
+ * corresponding paging entry in this order.
+ *
+ * The root page at PML4 does not participate in this indexing scheme, since
+ * it is statically allocated by pmap_pinit() and not by _pmap_allocpte().
*/
static vm_page_t
_pmap_allocpte(pmap_t pmap, vm_pindex_t ptepindex, struct rwlock **lockp)

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