don't let dirty blocks be evicted from cache!

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;

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)
 {

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!

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"