Skip missing page directories in deallocuvm

Previously, deallocuvm scanned from 0 to KERNBASE in one page increments, which had a noticable effect on boot time. Now it skips over missing page directories.

Skip missing page directories in deallocuvm
052e1848 · Austin Clements · 1eadf4a8 · 052e1848
--- a/vm.c
+++ b/vm.c
@@ -261,7 +261,9 @@ deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  a = PGROUNDUP(newsz);
  for(; a  < oldsz; a += PGSIZE){
    pte = walkpgdir(pgdir, (char*)a, 0);
-    if(pte && (*pte & PTE_P) != 0){
+    if(!pte)
+      a += (NPTENTRIES - 1) * PGSIZE;
+    else if((*pte & PTE_P) != 0){
      pa = PTE_ADDR(*pte);
      if(pa == 0)
        panic("kfree");