From 0b94a6a45fc79ad87112d16622bb5f2f130c893b Mon Sep 17 00:00:00 2001
From: Ivy Wu <ivy.wu.2003@gmail.com>
Date: Thu, 10 Oct 2024 22:49:32 -0400
Subject: [PATCH] update lab net

---
 grade-lab-net  |  4 +++
 user/nettest.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+)

diff --git a/grade-lab-net b/grade-lab-net
index 3b2707b..5018168 100755
--- a/grade-lab-net
+++ b/grade-lab-net
@@ -50,6 +50,10 @@ def test_nettest_():
 def test_nettest_():
     r.match('^dns: OK$')
 
+@test(10, "nettest: free", parent=test_nettest)
+def test_nettest_():
+    r.match('^free: OK$')
+
 #@test(10, "answers-net.txt")
 #def test_answers():
 #    # just a simple sanity check, will be graded manually
diff --git a/user/nettest.c b/user/nettest.c
index 2f4272f..19b39ff 100644
--- a/user/nettest.c
+++ b/user/nettest.c
@@ -830,6 +830,72 @@ usage()
   exit(1);
 }
 
+//
+// use sbrk() to count how many free physical memory pages there are.
+// touches the pages to force allocation.
+// because out of memory with lazy allocation results in the process
+// taking a fault and being killed, fork and report back.
+//
+int
+countfree()
+{
+  int fds[2];
+
+  if(pipe(fds) < 0){
+    printf("pipe() failed in countfree()\n");
+    exit(1);
+  }
+  
+  int pid = fork();
+
+  if(pid < 0){
+    printf("fork failed in countfree()\n");
+    exit(1);
+  }
+
+  if(pid == 0){
+    close(fds[0]);
+    
+    while(1){
+      uint64 a = (uint64) sbrk(4096);
+      if(a == 0xffffffffffffffff){
+        break;
+      }
+
+      // modify the memory to make sure it's really allocated.
+      *(char *)(a + 4096 - 1) = 1;
+
+      // report back one more page.
+      if(write(fds[1], "x", 1) != 1){
+        printf("write() failed in countfree()\n");
+        exit(1);
+      }
+    }
+
+    exit(0);
+  }
+
+  close(fds[1]);
+
+  int n = 0;
+  while(1){
+    char c;
+    int cc = read(fds[0], &c, 1);
+    if(cc < 0){
+      printf("read() failed in countfree()\n");
+      exit(1);
+    }
+    if(cc == 0)
+      break;
+    n += 1;
+  }
+
+  close(fds[0]);
+  wait((int*)0);
+  
+  return n;
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -856,6 +922,8 @@ main(int argc, char *argv[])
     //
     // "python3 nettest.py grade" must already be running...
     //
+    int free0 = countfree();
+    int free1 = 0;
     txone();
     sleep(2);
     ping0();
@@ -868,6 +936,11 @@ main(int argc, char *argv[])
     sleep(2);
     dns();
     sleep(2);
+    if ((free1 = countfree()) + 32 < free0) {
+      printf("free: FAILED -- lost too many free pages %d (out of %d)\n", free1, free0);
+    } else {
+      printf("free: OK\n");
+    }
   } else if(strcmp(argv[1], "dns") == 0){
     dns();
   } else {