do not allocate & zero pages until faulted,

to help measure just the map/unmap cost.

do not allocate & zero pages until faulted,
d871d353 · Nickolai Zeldovich · cb5aba0f · d871d353
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -26,10 +26,9 @@ vmnode::vmnode(u64 npg, vmntype ntype, inode *i, u64 off, u64 s)
  if (npg > NELEM(page))
    panic("vmnode too big\n");
  memset(page, 0, npg * sizeof(page[0]));
-  if (type == EAGER) {
+  if (type == EAGER && ip) {
    assert(allocpg() == 0);
-    if (ip)
-      assert(demand_load() == 0);
+    assert(demand_load() == 0);
  }
 }

@@ -416,12 +415,13 @@ vmap::pagefault(uptr va, u32 err)
    cprintf("pagefault: err 0x%x va 0x%lx type %d ref %lu pid %d\n",
            err, va, m->va_type, m->n->ref.load(), myproc()->pid);

-  if (m->n && m->n->type == ONDEMAND && m->n->page[npg] == 0) {
+  if (m->n && !m->n->page[npg])
    if (m->n->allocpg() < 0)
      panic("pagefault: couldn't allocate pages");
+
+  if (m->n && m->n->type == ONDEMAND)
    if (m->n->demand_load() < 0)
      panic("pagefault: couldn't load");
-  }

  if (m->va_type == COW && (err & FEC_WR)) {
    if (pagefault_wcow(m) < 0)
@@ -468,6 +468,7 @@ vmap::copyout(uptr va, void *p, u64 len)
    if(vma == 0)
      return -1;

+    vma->n->allocpg();
    uptr pn = (va0 - vma->vma_start) / PGSIZE;
    char *p0 = vma->n->page[pn];
    if(p0 == 0)