handle another out-of-memory in fork(). the policy here is not consistent,

since other calls (e.g. exec()) panic on out of memory.

kernel/proc.c

@@ -161,6 +161,8 @@ proc_pagetable(struct proc *p)
 
   // An empty page table.
   pagetable = uvmcreate();
+  if(pagetable == 0)
+    return 0;
 
   // map the trampoline code (for system call return)
   // at the highest user virtual address.

kernel/vm.c

@@ -195,13 +195,14 @@ uvmunmap(pagetable_t pagetable, uint64 va, uint64 npages, int do_free)
 }
 
 // create an empty user page table.
+// returns 0 if out of memory.
 pagetable_t
 uvmcreate()
 {
   pagetable_t pagetable;
   pagetable = (pagetable_t) kalloc();
   if(pagetable == 0)
-    panic("uvmcreate: out of memory");
+    return 0;
   memset(pagetable, 0, PGSIZE);
   return pagetable;
 }

user/usertests.c

@@ -2507,6 +2507,8 @@ main(int argc, char *argv[])
 
   if(argc == 2 && strcmp(argv[1], "-c") == 0){
     continuous = 1;
+  } else if(argc == 2 && strcmp(argv[1], "-C") == 0){
+    continuous = 2;
   } else if(argc == 2 && argv[1][0] != '-'){
     justone = argv[1];
   } else if(argc > 1){
@@ -2589,11 +2591,13 @@ main(int argc, char *argv[])
       }
       if(fail){
         printf("SOME TESTS FAILED\n");
+        if(continuous != 2)
           exit(1);
       }
       int free1 = countfree();
       if(free1 < free0){
         printf("FAILED -- lost some free pages\n");
+        if(continuous != 2)
           exit(1);
       }
     }