diff --git a/bio.c b/bio.c
index 88b0964..de1d0f2 100644
--- a/bio.c
+++ b/bio.c
@@ -79,9 +79,9 @@ bget(uint dev, uint sector)
     }
   }
 
-  // Not cached; recycle some existing buffer.
+  // Not cached; recycle some non-busy and clean buffer.
   for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
-    if((b->flags & B_BUSY) == 0){
+    if((b->flags & B_BUSY) == 0 && (b->flags & B_DIRTY) == 0){
       b->dev = dev;
       b->sector = sector;
       b->flags = B_BUSY;
diff --git a/defs.h b/defs.h
index 12f021b..23b1019 100644
--- a/defs.h
+++ b/defs.h
@@ -142,8 +142,8 @@ char*           strncpy(char*, const char*, int);
 int             argint(int, int*);
 int             argptr(int, char**, int);
 int             argstr(int, char**);
-int             fetchint(struct proc*, uint, int*);
-int             fetchstr(struct proc*, uint, char**);
+int             fetchint(uint, int*);
+int             fetchstr(uint, char**);
 void            syscall(void);
 
 // timer.c
diff --git a/file.c b/file.c
index 3fd2cbe..53c5af2 100644
--- a/file.c
+++ b/file.c
@@ -1,3 +1,7 @@
+//
+// File descriptors
+//
+
 #include "types.h"
 #include "defs.h"
 #include "param.h"
@@ -87,7 +91,7 @@ filestat(struct file *f, struct stat *st)
   return -1;
 }
 
-// Read from file f.  Addr is kernel address.
+// Read from file f.
 int
 fileread(struct file *f, char *addr, int n)
 {
@@ -108,7 +112,7 @@ fileread(struct file *f, char *addr, int n)
 }
 
 //PAGEBREAK!
-// Write to file f.  Addr is kernel address.
+// Write to file f.
 int
 filewrite(struct file *f, char *addr, int n)
 {
diff --git a/log.c b/log.c
index 5b827de..1814edc 100644
--- a/log.c
+++ b/log.c
@@ -177,6 +177,7 @@ log_write(struct buf *b)
   brelse(lbuf);
   if (i == log.lh.n)
     log.lh.n++;
+  b->flags |= B_DIRTY; // XXX prevent eviction
 }
 
 //PAGEBREAK!
diff --git a/syscall.c b/syscall.c
index 0918da7..45f758e 100644
--- a/syscall.c
+++ b/syscall.c
@@ -13,28 +13,28 @@
 // library system call function. The saved user %esp points
 // to a saved program counter, and then the first argument.
 
-// Fetch the int at addr from process p.
+// Fetch the int at addr from the current process.
 int
-fetchint(struct proc *p, uint addr, int *ip)
+fetchint(uint addr, int *ip)
 {
-  if(addr >= p->sz || addr+4 > p->sz)
+  if(addr >= proc->sz || addr+4 > proc->sz)
     return -1;
   *ip = *(int*)(addr);
   return 0;
 }
 
-// Fetch the nul-terminated string at addr from process p.
+// Fetch the nul-terminated string at addr from the current process.
 // Doesn't actually copy the string - just sets *pp to point at it.
 // Returns length of string, not including nul.
 int
-fetchstr(struct proc *p, uint addr, char **pp)
+fetchstr(uint addr, char **pp)
 {
   char *s, *ep;
 
-  if(addr >= p->sz)
+  if(addr >= proc->sz)
     return -1;
   *pp = (char*)addr;
-  ep = (char*)p->sz;
+  ep = (char*)proc->sz;
   for(s = *pp; s < ep; s++)
     if(*s == 0)
       return s - *pp;
@@ -45,7 +45,7 @@ fetchstr(struct proc *p, uint addr, char **pp)
 int
 argint(int n, int *ip)
 {
-  return fetchint(proc, proc->tf->esp + 4 + 4*n, ip);
+  return fetchint(proc->tf->esp + 4 + 4*n, ip);
 }
 
 // Fetch the nth word-sized system call argument as a pointer
@@ -74,7 +74,7 @@ argstr(int n, char **pp)
   int addr;
   if(argint(n, &addr) < 0)
     return -1;
-  return fetchstr(proc, addr, pp);
+  return fetchstr(addr, pp);
 }
 
 extern int sys_chdir(void);
diff --git a/sysfile.c b/sysfile.c
index 9de3d86..64f2b33 100644
--- a/sysfile.c
+++ b/sysfile.c
@@ -1,3 +1,9 @@
+//
+// File-system system calls.
+// Mostly argument checking, since we don't trust
+// user code, and calls into file.c and fs.c.
+//
+
 #include "types.h"
 #include "defs.h"
 #include "param.h"
@@ -382,13 +388,13 @@ sys_exec(void)
   for(i=0;; i++){
     if(i >= NELEM(argv))
       return -1;
-    if(fetchint(proc, uargv+4*i, (int*)&uarg) < 0)
+    if(fetchint(uargv+4*i, (int*)&uarg) < 0)
       return -1;
     if(uarg == 0){
       argv[i] = 0;
       break;
     }
-    if(fetchstr(proc, uarg, &argv[i]) < 0)
+    if(fetchstr(uarg, &argv[i]) < 0)
       return -1;
   }
   return exec(path, argv);