cowtest fixed and try to finish usertests

kernel/kalloc.c

@@ -9,6 +9,12 @@
 #include "riscv.h"
 #include "defs.h"
 
+#define NPAGE ((PHYSTOP - KERNBASE) / PGSIZE)
+static int refcnt[NPAGE];
+static inline int pa2idx(void *pa) {
+  return ((uint64)pa - KERNBASE) / PGSIZE;
+}
+
 void freerange(void *pa_start, void *pa_end);
 
 extern char end[]; // first address after kernel.
@@ -23,10 +29,25 @@ struct {
   struct run *freelist;
 } kmem;
 
+void incref(void *pa) {
+  int idx = pa2idx(pa);
+  refcnt[idx]++;
+}
+void decref(void *pa) {
+  int idx = pa2idx(pa);
+  refcnt[idx]--;
+}
+int getref(void *pa) {
+  int idx = pa2idx(pa);
+  return refcnt[idx];
+}
+
 void
 kinit()
 {
   initlock(&kmem.lock, "kmem");
+  for(int i = 0; i < NPAGE; i++)
+    refcnt[i] = 0;
   freerange(end, (void*)PHYSTOP);
 }
 
@@ -51,6 +72,16 @@ kfree(void *pa)
   if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
     panic("kfree");
 
+  int idx = pa2idx(pa);
+  acquire(&kmem.lock);
+  if(refcnt[idx] > 1) {
+    refcnt[idx]--;
+    release(&kmem.lock);
+    return;
+  }
+  refcnt[idx] = 0;
+  release(&kmem.lock);
+
   // Fill with junk to catch dangling refs.
   memset(pa, 1, PGSIZE);
 
@@ -76,7 +107,10 @@ kalloc(void)
     kmem.freelist = r->next;
   release(&kmem.lock);
 
-  if(r)
+  if(r) {
     memset((char*)r, 5, PGSIZE); // fill with junk
+    int idx = pa2idx((void*)r);
+    refcnt[idx] = 1;
+  }
   return (void*)r;
 }