Remove what I think is an incorrect assert from vm.cc.

In vmap::pagefault the assert assumed a PRIVATE vma would have a vmnode with 1 ref. In vmap::copy, however, the vmnode reference is incremented, then the vma is replaced with one of type COW. If a thread is pagefaulting while another is forking there is a race.

Remove what I think is an incorrect assert from vm.cc.
1efb8580 · Silas Boyd-Wickizer · ba0c5af6 · 1efb8580
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -546,7 +546,6 @@ vmap::pagefault(uptr va, u32 err)
  if (m->va_type == COW) {
    *pte = v2p(m->n->page[npg]) | PTE_P | PTE_U | PTE_COW;
  } else {
-    assert(m->n->ref == 1);
    *pte = v2p(m->n->page[npg]) | PTE_P | PTE_U | PTE_W;
  }